fix #15873, prevent FontFamily Property was null and crash the whole application #15896
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What does the pull request do?
this PR fix when FontFamily Property assign a {x:Null} value by xaml, will crash whole application, i added a null check for Typeface.FontFamily.
What is the current behavior?
FontManager will not check the null reference of Typeface.FontFamily null reference, if this property was null, will not fallback to DefaultFontFamily, and crash whole application. I think for the application robustness, it's should be checked.
What is the updated/expected behavior with this PR?
when Typeface.FontFamily property was null, it can fallback to default family
How was the solution implemented (if it's not obvious)?
Add a simple null-coalescing
Fixed issues
Fixes #15873