fix #12190, the name of FontFamily is null will cause crash

[JerryJian]

What does the pull request do?

This PR fix if the name of FontFamily is null when calling the constructor of FontFamily from Typeface, will cause the application crash.

What is the current behavior?

When the name of FontFamily is null, use FontFamily.Default instead.

What is the updated/expected behavior with this PR?

When the name of FontFamily is null, it can fallback to default family.

How was the solution implemented (if it's not obvious)?

Add a null check ternary conditional operator

Fixed issues

Fixes #12190

[avaloniaui-bot]

You can test this PR using the following package version. 11.2.999-cibuild0051218-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

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[JerryJian]

@cla-avalonia agree