[web] avoid calling worker with empty store ops

Summary: That was previously the way it worked, I removed it when refactoring which was not so good, so bringing it back.

Test Plan:

1. Thread, Draft and report ops worker.
2. Other are not scheduled on worker.

Reviewers: tomek, michal

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D10300