fix: Blazor ViewModel nullability during rendering

[RLittlesII]

What kind of change does this PR introduce?

Resolves an issue where the Blazor Component checking for a ViewModel property while rendering will cause an error

What is the current behavior?

It will setup a WhenAnyValue on the ViewModel in the constructor, before it's rendered and blow up.

What is the new behavior?

The WhenAny operator will only execute on the component after the component is rendered.

What might this PR break?

ReactiveUI.Blazor

Please check if the PR fulfills 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 #2336 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2336   +/-   ##
=======================================
  Coverage   54.73%   54.73%           
=======================================
  Files         113      113           
  Lines        4334     4334           
  Branches      660      660           
=======================================
  Hits         2372     2372           
  Misses       1797     1797           
  Partials      165      165

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