CmdPal: Null pattern matching based on is expression rather than overridable operators (#40972)

What the title says. 😄 

Rather than relying on the potentially overloaded `!=` or `==` operators
when checking for null, now we'll use the `is` expression (possibly
combined with the `not` operator) to ensure correct checking. Probably
overkill for many of these classes, but decided to err on the side of
consistency. Would matter more on classes that may be inherited or
extended.

Using `is` and `is not` will provide us a guarantee that no
user-overloaded equality operators (`==`/`!=`) is invoked when a
`expression is null` is evaluated.

In code form, changed all instances of:

```c#
something != null

something == null
```

to:

```c#
something is not null

something is null
```

The one exception was checking null on a `KeyChord`. `KeyChord` is a
struct which is never null so VS will raise an error when trying this
versus just providing a warning when using `keyChord != null`. In
reality, we shouldn't do this check because it can't ever be null. In
the case of a `KeyChord` it **would** be a `KeyChord` equivalent to:

```c#
KeyChord keyChord = new ()
{
    Modifiers = 0,
    Vkey = 0,
    ScanCode = 0
};
```

src/modules/cmdpal/Microsoft.CmdPal.UI/Helpers/GpoValueChecker.cs

@@ -2,13 +2,6 @@
 // The Microsoft Corporation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
-using System;
-using System.Collections.Generic;
-using System.Linq;
-using System.Text;
-using System.Threading.Tasks;
-using Microsoft.CmdPal.Ext.Bookmarks;
-using Microsoft.UI.Xaml.Documents;
 using Microsoft.Win32;
 
 namespace Microsoft.CmdPal.UI.Helpers;
@@ -63,7 +56,7 @@ internal static class GpoValueChecker
     {
         using (RegistryKey? key = rootKey.OpenSubKey(subKeyPath, false))
         {
-            if (key == null)
+            if (key is null)
             {
                 return null;
             }

src/modules/cmdpal/Microsoft.CmdPal.UI/Helpers/IconCacheProvider.cs

@@ -4,7 +4,6 @@
 
 using Microsoft.CmdPal.Core.ViewModels;
 using Microsoft.CmdPal.UI.Controls;
-using Microsoft.CmdPal.UI.Helpers;
 
 namespace Microsoft.CmdPal.UI.Helpers;
 
@@ -19,7 +18,7 @@ public static partial class IconCacheProvider
     public static async void SourceRequested(IconBox sender, SourceRequestedEventArgs args)
 #pragma warning restore IDE0060 // Remove unused parameter
     {
-        if (args.Key == null)
+        if (args.Key is null)
         {
             return;
         }

src/modules/cmdpal/Microsoft.CmdPal.UI/Helpers/IconCacheService.cs

@@ -28,7 +28,7 @@ public sealed class IconCacheService(DispatcherQueue dispatcherQueue)
                 var source = IconPathConverter.IconSourceMUX(icon.Icon, false);
                 return source;
             }
-            else if (icon.Data != null)
+            else if (icon.Data is not null)
             {
                 try
                 {
@@ -49,7 +49,7 @@ public sealed class IconCacheService(DispatcherQueue dispatcherQueue)
 
     private async Task<IconSource?> StreamToIconSource(IRandomAccessStreamReference iconStreamRef)
     {
-        if (iconStreamRef == null)
+        if (iconStreamRef is null)
         {
             return null;
         }

src/modules/cmdpal/Microsoft.CmdPal.UI/Helpers/TrayIconService.cs

@@ -50,7 +50,7 @@ internal sealed partial class TrayIconService
     {
         if (showSystemTrayIcon ?? _settingsModel.ShowSystemTrayIcon)
         {
-            if (_window == null)
+            if (_window is null)
             {
                 _window = new Window();
                 _hwnd = new HWND(WindowNative.GetWindowHandle(_window));
@@ -64,7 +64,7 @@ internal sealed partial class TrayIconService
                 _originalWndProc = Marshal.GetDelegateForFunctionPointer<WNDPROC>(PInvoke.SetWindowLongPtr(_hwnd, WINDOW_LONG_PTR_INDEX.GWL_WNDPROC, hotKeyPrcPointer));
             }
 
-            if (_trayIconData == null)
+            if (_trayIconData is null)
             {
                 // We need to stash this handle, so it doesn't clean itself up. If
                 // explorer restarts, we'll come back through here, and we don't
@@ -88,7 +88,7 @@ internal sealed partial class TrayIconService
             // Add the notification icon
             PInvoke.Shell_NotifyIcon(NOTIFY_ICON_MESSAGE.NIM_ADD, in d);
 
-            if (_popupMenu == null)
+            if (_popupMenu is null)
             {
                 _popupMenu = PInvoke.CreatePopupMenu_SafeHandle();
                 PInvoke.InsertMenu(_popupMenu, 0, MENU_ITEM_FLAGS.MF_BYPOSITION | MENU_ITEM_FLAGS.MF_STRING, PInvoke.WM_USER + 1, RS_.GetString("TrayMenu_Settings"));
@@ -103,7 +103,7 @@ internal sealed partial class TrayIconService
 
     public void Destroy()
     {
-        if (_trayIconData != null)
+        if (_trayIconData is not null)
         {
             var d = (NOTIFYICONDATAW)_trayIconData;
             if (PInvoke.Shell_NotifyIcon(NOTIFY_ICON_MESSAGE.NIM_DELETE, in d))
@@ -112,19 +112,19 @@ internal sealed partial class TrayIconService
             }
         }
 
-        if (_popupMenu != null)
+        if (_popupMenu is not null)
         {
             _popupMenu.Close();
             _popupMenu = null;
         }
 
-        if (_largeIcon != null)
+        if (_largeIcon is not null)
         {
             _largeIcon.Close();
             _largeIcon = null;
         }
 
-        if (_window != null)
+        if (_window is not null)
         {
             _window.Close();
             _window = null;
@@ -167,7 +167,7 @@ internal sealed partial class TrayIconService
             // WM_WINDOWPOSCHANGING which is always received on explorer startup sequence.
             case PInvoke.WM_WINDOWPOSCHANGING:
                 {
-                    if (_trayIconData == null)
+                    if (_trayIconData is null)
                     {
                         SetupTrayIcon();
                     }
@@ -189,7 +189,7 @@ internal sealed partial class TrayIconService
                     {
                         case PInvoke.WM_RBUTTONUP:
                             {
-                                if (_popupMenu != null)
+                                if (_popupMenu is not null)
                                 {
                                     PInvoke.GetCursorPos(out var cursorPos);
                                     PInvoke.SetForegroundWindow(_hwnd);

src/modules/cmdpal/Microsoft.CmdPal.UI/Helpers/TypedEventHandlerExtensions.cs

@@ -46,7 +46,7 @@ public static class TypedEventHandlerExtensions
 #pragma warning restore CA1715 // Identifiers should have correct prefix
         where R : DeferredEventArgs
     {
-        if (eventHandler == null)
+        if (eventHandler is null)
         {
             return Task.CompletedTask;
         }

src/modules/cmdpal/Microsoft.CmdPal.UI/Helpers/WindowHelper.cs

@@ -13,7 +13,7 @@ internal sealed partial class WindowHelper
         UserNotificationState state;
 
         // https://learn.microsoft.com/en-us/windows/win32/api/shellapi/ne-shellapi-query_user_notification_state
-        if (Marshal.GetExceptionForHR(NativeMethods.SHQueryUserNotificationState(out state)) == null)
+        if (Marshal.GetExceptionForHR(NativeMethods.SHQueryUserNotificationState(out state)) is null)
         {
             if (state == UserNotificationState.QUNS_RUNNING_D3D_FULL_SCREEN ||
                 state == UserNotificationState.QUNS_BUSY ||