Fix nullsafe FIXMEs for BlobModule.java and mark nullsafe (#50366)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/50366

Gone trough all the FIXMEs added in the previous diff by the nullsafe tool, marked the class as nullsafe and ensured no remaining violations.
Changelog: [Android][Fixed] Made BlobModule.java nullsafe

Reviewed By: cortinico

Differential Revision: D71979598

fbshipit-source-id: bbef5548d05e0b77ea03cf72f41a384fe7294d59