Fix MVVM Toolkit LINQ expression issues on .NET Framework

[Sergio0694]

Fixes #4272

PR Type

What kind of change does this PR introduce?

• Bugfix

What is the current behavior?

There are a couple of scenarios currently where the MVVM Toolkit crashes on .NET Framework:

• LINQ expression fallback path for ObservableValidator.ValidateAllProperties on a model with no validatable properties
• LINQ expression fallback path for IMessengerExtensions.RegisterAll on a recipient with no declared handlers

This is due to Expression.Block throwing when receiving an empty collection as input on .NET Framework.
It works fine from .NET Core 2.1 and onwards.

What is the new behavior?

Added an explicit check and short path to skip the LINQ expression calls entirely if no targets are detected.
Both scenarios work correctly now.

PR Checklist

Please check if your PR fulfills the following requirements:

• Tested code with current supported SDKs
• New component
  • Pull Request has been submitted to the documentation repository instructions. Link:
  • Added description of major feature to project description for NuGet package (4000 total character limit, so don't push entire description over that)
  • If control, added to Visual Studio Design project
• Sample in sample app has been added / updated (for bug fixes / features)
  • Icon has been created (if new sample) following the Thumbnail Style Guide and templates
• New major technical changes in the toolkit have or will be added to the Wiki e.g. build changes, source generators, testing infrastructure, sample creation changes, etc...
• Tests for the changes have been added (for bug fixes / features) (if applicable)
• Header has been added to all new source files (run build/UpdateHeaders.bat)
• Contains NO breaking changes

[ghost]

Thanks Sergio0694 for opening a Pull Request! The reviewers will test the PR and highlight if there is any conflict or changes required. If the PR is approved we will proceed to merge the pull request 🙌