CmdPal: Guard Frame.GoBack to prevent crash (#46493)

## Summary of the Pull Request

This PR adds a guard to the shell page that prevents navigating back
with empty nav stack (which leads to exception, an ultimately to a
crash).

> You should check that the
[CanGoBack](https://learn.microsoft.com/en-us/windows/windows-app-sdk/api/winrt/microsoft.ui.xaml.controls.frame.cangoback?view=windows-app-sdk-1.8#microsoft-ui-xaml-controls-frame-cangoback)
property is true before you call GoBack. If you call GoBack while
CanGoBack is false, an exception is thrown.

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## PR Checklist

- [x] Closes: #46492
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issues) -->
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- [ ] **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
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for new binaries
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- [ ] **Documentation updated:** If checked, please file a pull request
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and link it here: #xxx

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## Detailed Description of the Pull Request / Additional comments

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## Validation Steps Performed

src/modules/cmdpal/Microsoft.CmdPal.UI/Pages/ShellPage.xaml.cs

@@ -26,7 +26,6 @@ using Microsoft.UI.Xaml;
 using Microsoft.UI.Xaml.Controls;
 using Microsoft.UI.Xaml.Input;
 using Microsoft.UI.Xaml.Media.Animation;
-using Windows.UI.Core;
 using WinUIEx;
 using DispatcherQueue = Microsoft.UI.Dispatching.DispatcherQueue;
 using VirtualKey = Windows.System.VirtualKey;
@@ -485,6 +484,8 @@ public sealed partial class ShellPage : Microsoft.UI.Xaml.Controls.Page,
         // However, then we have more fine-grained control on the back stack, managing the VM cache, and not
         // having that all be a black box, though then we wouldn't cache the XAML page itself, but sometimes that is a drawback.
         // However, we do a good job here, see ForwardStack.Clear below, and BackStack.Clear above about managing that.
+        if (RootFrame.CanGoBack)
+        {
             if (withAnimation)
             {
                 RootFrame.GoBack();
@@ -499,6 +500,7 @@ public sealed partial class ShellPage : Microsoft.UI.Xaml.Controls.Page,
             // back to a recent page they visited (like the Pokedex) so we don't have to reload it from  scratch.
             // That'd be retrieved as we re-navigate in the PerformCommandMessage logic above
             RootFrame.ForwardStack.Clear();
+        }
 
         if (!RootFrame.CanGoBack)
         {