MM-42282: handle teamId parameter correctly (#19685)

As per https://community-daily.mattermost.com/core/pl/ugs7ue6e4j8a7cgegk1bxje8to, `ThreadStore.GetThreadsForUser` accepts a `teamId` parameter, but incorrectly handles an empty value of `""` as looking only for channels with an empty `teamId` (aka DMs and GMs) instead of finding all channels and effectively ignoring the team property.

Fixes: https://mattermost.atlassian.net/browse/MM-42282

store/sqlstore/thread_store.go

@@ -67,7 +67,16 @@ func (s *SqlThreadStore) GetThreadsForUser(userId, teamId string, opts model.Get
 	fetchConditions := sq.And{
 		sq.Eq{"ThreadMemberships.UserId": userId},
 		sq.Eq{"ThreadMemberships.Following": true},
-		sq.Or{sq.Eq{"Channels.TeamId": teamId}, sq.Eq{"Channels.TeamId": ""}},
+	}
+
+	if teamId != "" {
+		fetchConditions = sq.And{
+			fetchConditions,
+			sq.Or{
+				sq.Eq{"Channels.TeamId": teamId},
+				sq.Eq{"Channels.TeamId": ""},
+			},
+		}
 	}
 	if !opts.Deleted {
 		fetchConditions = sq.And{