[native] fix empty media gallery in thread settings

Summary:
We had a bug in our thread settings where we would show ui elements for the media gallery even if no items were present. This diff fixes that

Linear task: https://linear.app/comm/issue/ENG-3301/[native]-empty-media-gallery-looks-out-of-place

Test Plan:
Please see the demo video below

Reviewers: inka, ashoat

Reviewed By: ashoat

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12581