fix for WhenAny WhenAnyValue ToProperty

[ChrisPulman]

What kind of change does this PR introduce?

update WhenAny WhenAnyValue to use nullable for selector
fix #2812 ToProperty to work with nullable OAPH and non nullable OAPH

What is the current behaviour?

causes compiler issue or runtime exception

What is the new behaviour?

WhenAnyValue and ToProperty can be used in conjunction with each other without compiler warnings or runtime exceptions.

What might this PR break?

nullablilty of OAPH needs to be checked.

Please check if the PR fulfils these requirements

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

Other information:

[codecov]

Codecov Report

Merging #2860 (f77da5f) into main (259f4aa) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #2860      +/-   ##
==========================================
+ Coverage   71.81%   71.82%   +0.01%     
==========================================
  Files         137      137              
  Lines        4828     4824       -4     
==========================================
- Hits         3467     3465       -2     
+ Misses       1361     1359       -2     

Impacted Files	Coverage Δ
...eUI.Fody.Helpers/ObservableAsPropertyExtensions.cs	41.93% <ø> (ø)
...I/ObservableForProperty/OAPHCreationHelperMixin.cs	38.40% <ø> (ø)
src/ReactiveUI/VariadicTemplates.cs	53.36% <ø> (ø)
...iveUI/Mixins/ReactiveNotifyPropertyChangedMixin.cs	85.71% <100.00%> (+2.38%)	⬆️

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[github-actions]

This pull request has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.