Fix handling of empty $uids argument in change_flag()

program/lib/Roundcube/rcube_imap_cache.php

@@ -485,7 +485,7 @@ class rcube_imap_cache
             .", flags = flags ".($enabled ? "+ $idx" : "- $idx")
             ." WHERE user_id = ?"
                 ." AND mailbox = ?"
-                .($uids !== null ? " AND uid IN (".$this->db->array2list($uids, 'integer').")" : "")
+                .(!empty($uids) ? " AND uid IN (".$this->db->array2list($uids, 'integer').")" : "")
                 ." AND (flags & $idx) ".($enabled ? "= 0" : "= $idx"),
             $this->userid, $mailbox);
     }