[web] Fix warnings on search screen

Summary:
issue: https://linear.app/comm/issue/ENG-4897/[web]-warnings-in-message-search-modal
when I did the following:

1. Search for a word
2. Scroll down
3. Clear the search query

I was seeing warnings on the console:

socket sent error data isn't of type T
erverError: data isn't of type T
    at InflightRequests.resolveRequestsForMessage (inflight-requests.js:142:139)
    at WebSocket.eval (socket.react.js:170:24)

This was because when the query was cleared queries.current[threadID] was undefined, instead of an empty string. So code in useSearchMessages didn't handle it correctly. This is fixed by this diff, as getQuery returns
queries.current[threadID] ?? ''. useSearchMessages then decieds not to call the server, but return right away.

Test Plan: checked that in the scenario decribed in the summary the error does not appear. Checked that it is possible to fetch messages, and loda more by scrollng down.

Reviewers: ginsu, rohan, michal, atul

Reviewed By: michal

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D9123