[lib] Fix type of messageInfoSelector

Summary: I noticed that createMessageInfo can return null, but the type messageInfoSelector did not consider null values. This diff just fixes the types up to be correct. (Luckily there were no issues that the fixed types exposed.)

Test Plan: Flow

Reviewers: atul

Reviewed By: atul

Subscribers: KatPo, palys-swm, Adrian

Differential Revision: https://phabricator.ashoat.com/D1426