[Flow202][web][skip-ci] [17/x] Fix Flow issues in commReduxStorageEngine
Summary:
The new version of Flow does not realize that typeof result.item !== 'string' will be false if result isn't set, so we need to add a separate check for this.
NOTE: CI will fail on this diff. I considered the possibility of fixing Flow errors BEFORE upgrading Flow, but it wasn't possible... in some cases, the fixes to support the new version of Flow caused errors in the old version. I could have hidden these type errors with $FlowFixMe lines and then later revert those, but that seemed like too much busy work.
Depends on D9782
Test Plan: Confirm the Flow errors go away
Reviewers: kamil, michal
Reviewed By: kamil
Subscribers: tomek, wyilio
Differential Revision: https://phab.comm.dev/D9783