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)
for new binaries and localization folder
- [ ] [YML for CI
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ci/templates/build-powertoys-steps.yml)
for new test projects
- [ ] [YML for signed
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/release.yml)
- [ ] **Documentation updated:** If checked, please file a pull request
on [our docs
repo](https://github.com/MicrosoftDocs/windows-uwp/tree/docs/hub/powertoys)
and link it here: #xxx

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## Detailed Description of the Pull Request / Additional comments

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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.AreEqual(Resources.remotedesktop_command_open, command.Name);
-        Assert.AreEqual(string.Empty, GetCommandHost(command));
+        Assert.IsNull(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.AreEqual(Resources.remotedesktop_command_open, command.Name);
-        Assert.AreEqual(string.Empty, GetCommandHost(command));
+        Assert.IsNull(command);
     }
 
     private static string GetCommandHost(OpenRemoteDesktopCommand command)