[keyserver] Make sure we don't call fetchServerThreadInfos with empty…

… threadIDs

Summary:
This diff fixes a regression introduced in D8509. We should never specify an empty `threadIDs` parameter to the `FetchThreadInfosFilter` we pass to `fetchServerThreadInfos`.

This diff addresses an urgent regression – see [ENG-4529](https://linear.app/comm/issue/ENG-4529/sql-error-breaking-message-sends) for details.

Test Plan: I confirmed that created a message in a thread with an @-tag of somebody who is already in the thread does not cause the error described in the linked task

Reviewers: atul, tomek, michal, bartek, kamil, inka, ginsu, varun

Reviewed By: tomek

Differential Revision: https://phab.comm.dev/D8651

keyserver/src/fetchers/thread-fetchers.js

@@ -78,6 +78,10 @@ type FetchServerThreadInfosResult = {
 async function fetchServerThreadInfos(
   filter?: FetchThreadInfosFilter,
 ): Promise<FetchServerThreadInfosResult> {
+  if (filter?.threadIDs?.size === 0) {
+    return { threadInfos: {} };
+  }
+
   let primaryFetchClause;
   if (filter?.accessibleToUserID) {
     primaryFetchClause = SQL`

keyserver/src/updaters/thread-permission-updaters.js

@@ -1074,6 +1074,7 @@ async function commitMembershipChangeset(
       });
     }
   }
+
   for (const row of membershipRowMap.values()) {
     const { userID, threadID } = row;
     if (row.operation === 'delete' || row.role === '-1') {