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) -->
- [ ] **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
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pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ci/templates/build-powertoys-steps.yml)
for new test projects
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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/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;
@@ -485,20 +484,23 @@ 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 (withAnimation)
+        if (RootFrame.CanGoBack)
         {
-            RootFrame.GoBack();
-        }
-        else
-        {
-            RootFrame.GoBack(_noAnimation);
-        }
+            if (withAnimation)
+            {
+                RootFrame.GoBack();
+            }
+            else
+            {
+                RootFrame.GoBack(_noAnimation);
+            }
 
-        // Don't store pages we're navigating away from in the Frame cache
-        // TODO: In the future we probably want a short cache (3-5?) of recent VMs in case the user re-navigates
-        // 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();
+            // Don't store pages we're navigating away from in the Frame cache
+            // TODO: In the future we probably want a short cache (3-5?) of recent VMs in case the user re-navigates
+            // 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)
         {