fix RoutedViewHost and ViewModelViewHost to allow DefaultContent

[ChrisPulman]

Refactored RoutedViewHost and ViewModelViewHost to allow DefaultContent to be shown

fixes #2815

What kind of change does this PR introduce?

fix

What is the current behaviour?

DefaultContent is not displayed for RoutedViewHost and ViewModelViewHost when loaded with null ViewModel

What is the new behaviour?

DefaultContent is displayed for RoutedViewHost and ViewModelViewHost when loaded with null ViewModel

What might this PR break?

Content will have a value when previously may not have depending upon the version of ReactiveUI i.e. >9.8.9

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 #2816 (9c5df4e) into main (ec9535e) will increase coverage by 2.48%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##             main    #2816      +/-   ##
==========================================
+ Coverage   65.77%   68.26%   +2.48%     
==========================================
  Files         137      137              
  Lines        4810     4787      -23     
==========================================
+ Hits         3164     3268     +104     
+ Misses       1646     1519     -127     

Impacted Files	Coverage Δ
...veUI/Platforms/windows-common/ViewModelViewHost.cs	83.33% <78.94%> (+74.76%)	⬆️
...ctiveUI/Platforms/windows-common/RoutedViewHost.cs	86.44% <86.95%> (+86.44%)	⬆️
src/ReactiveUI.Wpf/TransitioningContentControl.cs	12.72% <0.00%> (+3.63%)	⬆️
...UI/Platforms/windows-common/ReactiveUserControl.cs	72.72% <0.00%> (+72.72%)	⬆️

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[github-actions]

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