feature: Allow the ViewModel to be Null in BindValidation

[worldbeater]

What kind of change does this PR introduce?

This PR changes our nullability annotations to allow passing null view models to BindValidation. This could be potentially useful in cases when the view model is initialized after the control is activated, or when a view model is replaced with another view model after activation etc.

What is the current behavior?

Currently, we don't use the viewModel parameter in BindValidation, but check if it is null and throw ANE if it is null.

What is the new behavior?

Now, we support delayed view model initialization and just don't throw when BindValidation is called.

What might this PR break?

Nothing.

[codecov]

Codecov Report

Merging #152 into main will increase coverage by 0.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #152      +/-   ##
==========================================
+ Coverage   62.06%   62.14%   +0.07%     
==========================================
  Files          16       16              
  Lines         912      906       -6     
==========================================
- Hits          566      563       -3     
+ Misses        346      343       -3     

Impacted Files	Coverage Δ
Extensions/ViewForExtensions.cs	61.53% <0.00%> (+3.64%)	⬆️

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[worldbeater]

Added null guards where appropriate to Android extension methods for consistency with platform-agnostic BindValidations.