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/ext/Microsoft.CmdPal.Ext.ClipboardHistory/Helpers/ClipboardHelper.cs

@@ -139,7 +139,7 @@ internal static class ClipboardHelper
         switch (clipboardFormat)
         {
             case ClipboardFormat.Text:
-                if (clipboardItem.Content == null)
+                if (clipboardItem.Content is null)
                 {
                     ExtensionHost.LogMessage(new LogMessage() { Message = "No valid clipboard content" });
                     return;
@@ -152,7 +152,7 @@ internal static class ClipboardHelper
                 break;
 
             case ClipboardFormat.Image:
-                if (clipboardItem.ImageData == null)
+                if (clipboardItem.ImageData is null)
                 {
                     ExtensionHost.LogMessage(new LogMessage() { Message = "No valid clipboard content" });
                     return;
@@ -240,7 +240,7 @@ internal static class ClipboardHelper
     internal static async Task<SoftwareBitmap?> GetClipboardImageContentAsync(DataPackageView clipboardData)
     {
         using var stream = await GetClipboardImageStreamAsync(clipboardData);
-        if (stream != null)
+        if (stream is not null)
         {
             var decoder = await BitmapDecoder.CreateAsync(stream);
             return await decoder.GetSoftwareBitmapAsync();
@@ -255,7 +255,7 @@ internal static class ClipboardHelper
         {
             var storageItems = await clipboardData.GetStorageItemsAsync();
             var file = storageItems.Count == 1 ? storageItems[0] as StorageFile : null;
-            if (file != null)
+            if (file is not null)
             {
                 return await file.OpenReadAsync();
             }

src/modules/cmdpal/ext/Microsoft.CmdPal.Ext.ClipboardHistory/Models/ClipboardItem.cs

@@ -37,7 +37,7 @@ public class ClipboardItem
     }
 
     [MemberNotNullWhen(true, nameof(ImageData))]
-    private bool IsImage => ImageData != null;
+    private bool IsImage => ImageData is not null;
 
     [MemberNotNullWhen(true, nameof(Content))]
     private bool IsText => !string.IsNullOrEmpty(Content);

src/modules/cmdpal/ext/Microsoft.CmdPal.Ext.ClipboardHistory/Pages/ClipboardHistoryListPage.cs

@@ -54,7 +54,7 @@ internal sealed partial class ClipboardHistoryListPage : ListPage
         try
         {
             var allowClipboardHistory = Registry.GetValue(registryKey, "AllowClipboardHistory", null);
-            return allowClipboardHistory != null ? (int)allowClipboardHistory == 0 : false;
+            return allowClipboardHistory is not null ? (int)allowClipboardHistory == 0 : false;
         }
         catch (Exception)
         {
@@ -100,7 +100,7 @@ internal sealed partial class ClipboardHistoryListPage : ListPage
                 {
                     var imageReceived = await item.Item.Content.GetBitmapAsync();
 
-                    if (imageReceived != null)
+                    if (imageReceived is not null)
                     {
                         item.ImageData = imageReceived;
                     }
@@ -141,7 +141,7 @@ internal sealed partial class ClipboardHistoryListPage : ListPage
         for (var i = 0; i < clipboardHistory.Count; i++)
         {
             var item = clipboardHistory[i];
-            if (item != null)
+            if (item is not null)
             {
                 listItems.Add(item.ToListItem());
             }