Fix bindings without property path

[MrJul]

What does the pull request do?

This PR fixes bindings that are directly bound to a data context without any property path (e.g. {Binding}).

#13970 / v11.1 introduced a subtle change:

• In v11.0, a null data context is considered unset, whether there's a binding path or not.
• In v11.1, a null data context is considered unset when there's a binding path, or a valid null value if there isn't one.

While both results make sense, this is a behavioral breaking change from v11.0:

• OneTime bindings when the data context isn't known yet don't work anymore.
• NullTargetValue and StringFormat are being applied to the null data context.

This PR restores the 11.0 behavior.
Unit tests have been added.

For the rare cases where binding directly to a null data context is effectively expected (with a TargetNullValue set), we could add a new binding option in the future.

Fixed issues

• Fixes Binding OneTime does not work #16618

[avaloniaui-bot]

You can test this PR using the following package version. 11.2.999-cibuild0051356-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[workgroupengineering]

I tried it and it works as expected. Will it be backported or will the fix be released with 11.2?

[avaloniaui-bot]

You can test this PR using the following package version. 11.2.999-cibuild0051791-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]