Add DoNothingForNullConverter to Fix Color Palette Binding

[robloo]

What does the pull request do?

Fixes binding of the nullable selected palette color with the non-nullable Color property within the ColorView.

This issue was first discovered in the Windows Community Toolkit:

• ColorPickerButton Binding errors CommunityToolkit/WindowsCommunityToolkit#4721
• fix: Fix color picker button binding failure issue CommunityToolkit/WindowsCommunityToolkit#4730

What is the current behavior?

Avalonia behaves differently than WinUI/UWP. Instead of just throwing binding errors, Avalonia will clear the color (set it to the default #00000000 when null is attempted to be set).

This results in the color disappearing when opening with the palette tab selected.

Issue.mp4

What is the updated/expected behavior with this PR?

This PR fixes the issue of reset colors by simply ignoring binding updates when null is encountered in this situation. The simple DoNothingForNullConverter enables binding of a non-nullable with nullable property in this scenario.

Fix.mp4

Checklist

• Added unit tests (if possible)?
• Added XML documentation to any related classes?
• Consider submitting a PR to https://github.com/AvaloniaUI/Documentation with user documentation

Breaking changes

None

Obsoletions / Deprecations

None

Fixed issues

None opened for this

[avaloniaui-team]

You can test this PR using the following package version. 11.0.999-cibuild0023406-beta. (feed url: https://nuget.avaloniaui.net/repository/avalonia-all/index.json) [PRBUILDID]

[robloo]

This PR shouldn't break anything and would be good to merge as a non-breaking bug fix. I notice you are only merging fixes right now.

[avaloniaui-team]

You can test this PR using the following package version. 11.0.999-cibuild0023644-beta. (feed url: https://nuget.avaloniaui.net/repository/avalonia-all/index.json) [PRBUILDID]