Make ReactAccessibilityDelegate nullsafe (#53068)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/53068

Adds some assertion annotations and small null checks to `ReactAccessibilityDelegate` to comply with FB's Nullsafe annotation

Changelog: [Internal]

Reviewed By: arushikesarwani94

Differential Revision: D79645332

fbshipit-source-id: 707e4e6d4a5f09232af168c2f7c57c2fdbb1f08d