fix: align DepObjCol indexer behavior with winui (backport #19252)

[mergify]

GitHub Issue (If applicable): closes #19253, closes unoplatform/kahua-private#256

PR Type

What kind of change does this PR introduce?

• Bugfix

What is the current behavior?

On Uno, new BehaviorCollection()[2] throws:

System.ArgumentOutOfRangeException: Index was out of range. Must be non-negative and less than the size of the collection. (Parameter 'index')

What is the new behavior?

It will return null like on WinUI.

PR Checklist

Please check if your PR fulfills the following requirements:

• Docs have been added/updated which fit documentation template (for bug fixes / features)
• Unit Tests and/or UI Tests for the changes have been added (for bug fixes / features) (if applicable)
• Validated PR Screenshots Compare Test Run results.
• Contains NO breaking changes
• Associated with an issue (GitHub or internal) and uses the automatic close keywords.
• Commits must be following the Conventional Commits specification.

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  This is an automatic backport of pull request fix: align DepObjCol indexer behavior with winui #19252 done by Mergify.