Fix ManagedStorageProvider OverwritePrompt behaviour

[FishEagle]

What does the pull request do?

Fix issue with Avalonia.Dialogs.ManagedStorageProvider where it returns null, instead of the file selected, when the user choose to overwrite an existing file. As reported here: #17959

What is the current behavior?

Discards filename when the OverwritePrompt result is true.

What is the updated/expected behavior with this PR?

Returns filename so that SaveFilePickerAsync() result can be built correctly.

To test:

1. Ensure you are using Managed System Dialogs (e.g. AppBuilder.UseManagedSystemDialogs() )
2. var file = await storageProvider.SaveFilePickerAsync(...) should return the file selected to be overwritten, instead of null

Checklist

• Added unit tests (if possible)? The fix is fairly straightforward. I tried to implement tests, but could not quite get it working for the dialog - would appreciate guidance on whether it is necessary and how to do it for this component, if needed.
• Added XML documentation to any related classes? N/A
• Consider submitting a PR to https://github.com/AvaloniaUI/avalonia-docs with user documentation N/A

Breaking changes

None

Obsoletions / Deprecations

None

Fixed issues

Fixes #17959

[avaloniaui-bot]

You can test this PR using the following package version. 11.3.999-cibuild0054609-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[cla-avalonia]

• All contributors have signed the CLA.

[FishEagle]

@cla-avalonia agree

[avaloniaui-bot]

You can test this PR using the following package version. 11.3.999-cibuild0054631-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[maxkatz6]

Thanks!