[keyserver] Improve Flow types in update-creator

Summary:
We had a type error and Flow was unable to detect that. This diff makes it so the type error would be reported and replaces false with null to satisfy type system. In fetchMessageInfos and parseMessageSelectionCriteria we're using threadCursors[threadID] in if conditions so it is safe to do that replacement.

https://linear.app/comm/issue/ENG-4592/improve-flow-types-in-update-creator

Test Plan: Console log messageInfoResult before and after change and check if the result remains correct.

Reviewers: kamil, bartek, inka

Reviewed By: kamil

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D8768