fix(windows): clear video source when src is empty

[vivekjm]

Summary

Fixes #3188.

Motivation

On Windows, setting <Video source={null} /> after a video has already loaded should clear the current source. Instead, the Windows view manager only handled non-null src updates, and an empty uri could still be passed through to the WinRT Uri constructor. That leaves the player in a bad state and can crash when the app is trying to stop playback.

Changes

• Add a small Windows helper to pause the player and clear its media source
• Treat src: null, missing uri, and empty uri as a clear-source update
• Keep the existing non-empty URI path unchanged

Test plan

• Checked the Windows prop flow locally against the reported source={null} case
• Reviewed the diff to keep the change scoped to Windows source handling

I do not have a Windows dev environment available in this macOS workspace, so I was not able to run the native Windows sample build locally.