[native] Get rid of null check in useUploadSelectedMedia

Summary: We moved handling of null selection "out" to the callsite. This is no longer necessary.

Test Plan: Open image gallery via EditAvatar and then dismiss it and observe that we return before reaching useUploadSelectedMedia. Also flow.

Reviewers: ginsu, ashoat

Reviewed By: ginsu

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7703