CmdPal: Hide RDC fallback item by default (#43994)

## Summary of the Pull Request

This fixes few nits with RDC extension:
- hides the RDC fallback item from the home page when there’s no query;
- fixes MSTSC process working directory (must physically exists or be an
empty string)

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## PR Checklist

- [ ] Closes: #xxx
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issues) -->
- [ ] **Communication:** I've discussed this with core contributors
already. If the work hasn't been agreed, this work might be rejected
- [ ] **Tests:** Added/updated and all pass
- [ ] **Localization:** All end-user-facing strings can be localized
- [ ] **Dev docs:** Added/updated
- [ ] **New binaries:** Added on the required places
- [ ] [JSON for
signing](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ESRPSigning_core.json)
for new binaries
- [ ] [WXS for
installer](https://github.com/microsoft/PowerToys/blob/main/installer/PowerToysSetup/Product.wxs)
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- [ ] **Documentation updated:** If checked, please file a pull request
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repo](https://github.com/MicrosoftDocs/windows-uwp/tree/docs/hub/powertoys)
and link it here: #xxx

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or any additional comments/features here -->
## Detailed Description of the Pull Request / Additional comments

<!-- Describe how you validated the behavior. Add automated tests
wherever possible, but list manual validation steps taken as well -->
## Validation Steps Performed

src/modules/cmdpal/Tests/Microsoft.CmdPal.Ext.RemoteDesktop.UnitTests/FallbackRemoteDesktopItemTests.cs

@@ -73,13 +73,11 @@ public class FallbackRemoteDesktopItemTests
         fallback.UpdateQuery("   ");
 
         // Assert
-        Assert.AreEqual(Resources.remotedesktop_command_open, fallback.Title);
+        Assert.AreEqual(string.Empty, fallback.Title);
         Assert.AreEqual(string.Empty, fallback.Subtitle);
 
         var command = fallback.Command as OpenRemoteDesktopCommand;
-        Assert.IsNotNull(command);
+        Assert.IsNull(command);
-        Assert.AreEqual(Resources.remotedesktop_command_open, command.Name);
-        Assert.AreEqual(string.Empty, GetCommandHost(command));
     }
 
     [TestMethod]
@@ -93,13 +91,11 @@ public class FallbackRemoteDesktopItemTests
         fallback.UpdateQuery("not a valid host");
 
         // Assert
-        Assert.AreEqual(Resources.remotedesktop_command_open, fallback.Title);
+        Assert.AreEqual(string.Empty, fallback.Title);
         Assert.AreEqual(string.Empty, fallback.Subtitle);
 
         var command = fallback.Command as OpenRemoteDesktopCommand;
-        Assert.IsNotNull(command);
+        Assert.IsNull(command);
-        Assert.AreEqual(Resources.remotedesktop_command_open, command.Name);
-        Assert.AreEqual(string.Empty, GetCommandHost(command));
     }
 
     private static string GetCommandHost(OpenRemoteDesktopCommand command)

src/modules/cmdpal/ext/Microsoft.CmdPal.Ext.RemoteDesktop/Commands/FallbackRemoteDesktopItem.cs

@@ -23,13 +23,16 @@ internal sealed partial class FallbackRemoteDesktopItem : FallbackCommandItem
         ];
 
     private static readonly CompositeFormat RemoteDesktopOpenHostFormat = CompositeFormat.Parse(Resources.remotedesktop_open_host);
+
     private readonly IRdpConnectionsManager _rdpConnectionsManager;
+    private readonly NoOpCommand _emptyCommand = new NoOpCommand();
 
     public FallbackRemoteDesktopItem(IRdpConnectionsManager rdpConnectionsManager)
-    : base(new OpenRemoteDesktopCommand(string.Empty), Resources.remotedesktop_title)
+    : base(Resources.remotedesktop_title)
     {
         _rdpConnectionsManager = rdpConnectionsManager;
 
+        Command = _emptyCommand;
         Title = string.Empty;
         Subtitle = string.Empty;
         Icon = Icons.RDPIcon;
@@ -41,7 +44,7 @@ internal sealed partial class FallbackRemoteDesktopItem : FallbackCommandItem
         {
             Title = string.Empty;
             Subtitle = string.Empty;
-            Command = new OpenRemoteDesktopCommand(string.Empty);
+            Command = _emptyCommand;
             return;
         }
 
@@ -68,7 +71,7 @@ internal sealed partial class FallbackRemoteDesktopItem : FallbackCommandItem
         {
             Title = string.Empty;
             Subtitle = string.Empty;
-            Command = new OpenRemoteDesktopCommand(string.Empty);
+            Command = _emptyCommand;
         }
     }
 }

src/modules/cmdpal/ext/Microsoft.CmdPal.Ext.RemoteDesktop/Commands/OpenRemoteDesktopCommand.cs

@@ -40,7 +40,7 @@ internal sealed partial class OpenRemoteDesktopCommand : BaseObservable, IInvoka
     {
         using var process = new Process();
         process.StartInfo.UseShellExecute = false;
-        process.StartInfo.WorkingDirectory = Environment.SpecialFolder.MyDocuments.ToString();
+        process.StartInfo.WorkingDirectory = Environment.GetFolderPath(Environment.SpecialFolder.MyDocuments);
         process.StartInfo.FileName = "mstsc";
 
         if (!string.IsNullOrWhiteSpace(_rdpHost))

src/modules/cmdpal/extensionsdk/Microsoft.CommandPalette.Extensions.Toolkit/FallbackCommandItem.cs

@@ -6,7 +6,12 @@ namespace Microsoft.CommandPalette.Extensions.Toolkit;
 
 public partial class FallbackCommandItem : CommandItem, IFallbackCommandItem, IFallbackHandler
 {
-    private IFallbackHandler? _fallbackHandler;
+    private readonly IFallbackHandler? _fallbackHandler;
+
+    public FallbackCommandItem(string displayTitle)
+    {
+        DisplayTitle = displayTitle;
+    }
 
     public FallbackCommandItem(ICommand command, string displayTitle)
         : base(command)