[lib] Don't query keyserver for empty string from useThreadListSearch

Summary:
D11799 accidentally introduced a regression when replacing return [] that was previously here. It was replaced with a call to filterAndSetUserResults, but the return was accidentally dropped, causing us to query the keyserver for the empty string from here. This happens in the background repeatedly as the keyserver connection status changes.

This addresses ENG-8777

Depends on D12680

Test Plan:
I added some logs: https://gist.github.com/Ashoat/4ac8c57b3359b5a33fb8ee10268ada90

They allowed me to confirm that the empty string queries were no longer going through

Reviewers: inka, tomek

Reviewed By: inka, tomek

Differential Revision: https://phab.comm.dev/D12681