TwitterBridge: don't array_merge() non-arrays

Fixes: ErrorException: [2] array_merge(): Argument #2 is not an array

plugins/TwitterBridge/daemons/synctwitterfriends.php

@@ -183,9 +183,11 @@ class SyncTwitterFriendsDaemon extends ParallelizingDaemon
                            "of Twitter user $flink->foreign_id friends.");
                 continue;
             } else {
+                if (is_array($more_friends)) {
                     $friends = array_merge($friends, $more_friends);
                 }
             }
+        }
 
         return $friends;
     }