Fix IsNullOrEmptyStateTrigger for Uno to consider FEs as non-empty

[Youssef1313]

Fixes

Fixes unoplatform/uno#17311

PR Type

What kind of change does this PR introduce?

What is the current behavior?

FrameworkElements in Uno are incorrectly IEnumerables, so they could be considered as empty incorrectly.

What is the new behavior?

PR Checklist

Please check if your PR fulfills the following requirements:

• Created a feature/dev branch in your fork (vs. submitting directly from a commit on main)
• Based off latest main branch of toolkit
• Tested code with current supported SDKs
• New component
  • Documentation has been added
  • Sample in sample app has been added
  • Analyzers are passing for documentation and samples
  • Icon has been created (if new sample) following the Thumbnail Style Guide and templates
• Tests for the changes have been added (if applicable)
• Header has been added to all new source files
• Contains NO breaking changes

Other information

[Arlodotexe]

Thanks for the contribution @Youssef1313, I've added tracking details in a code comment so we can cleanup the workaround once the regression is fixed.

[Arlodotexe]

Seems these changes don't build as expected, here's the compiler error from CI:

         /home/runner/work/Windows/Windows/components/Triggers/src/IsNullOrEmptyStateTrigger.cs(121,32): error CS0128: A local variable or function named 'valEnumerable' is already defined in this scope [/home/runner/work/Windows/Windows/components/Triggers/src/CommunityToolkit.WinUI.Triggers.csproj::TargetFramework=net8.0]
         /home/runner/work/Windows/Windows/components/Triggers/src/IsNullOrEmptyStateTrigger.cs(127,34): error CS8602: Dereference of a possibly null reference. [/home/runner/work/Windows/Windows/components/Triggers/src/CommunityToolkit.WinUI.Triggers.csproj::TargetFramework=net8.0]

[agneszitte]

Thank you for the review @Arlodotexe!

[agneszitte]

@Arlodotexe, @Youssef1313 it seems that some tests are failing

[Youssef1313]

The failure looks unrelated to the change