[native] check for null fid before adding to sqlite database

Summary: This checks for a null fid before dispatching a setSyncedMetadataEntryActionType (which always expects a string).

Test Plan:
Went through new registration workflow on local dev environment and everything is working properly

Context: https://linear.app/comm/issue/ENG-7740/ios-simulator-app-crashing-a-brief-moment-after-completing-new#comment-11df0e2a

Reviewers: ginsu, ashoat, marcin, atul

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D11673