dnd: Don't pass null to ClutterActor.disconnect()

_ungrabActor disconnects the event signal handler but does not
check whether it is connected before doing so which can result into
an exception like:

JS ERROR: !!!   Exception was: Error: disconnect() takes one arg, the signal handler id
JS ERROR: !!!     lineNumber = '0'
JS ERROR: !!!     fileName = 'gjs_throw'
JS ERROR: !!!     stack = 'Error("disconnect() takes one arg, the signal handler id")@:0

js/ui/dnd.js

@@ -144,6 +144,8 @@ _Draggable.prototype = {
 
     _ungrabActor: function() {
         Clutter.ungrab_pointer();
+        if (!this._onEventId)
+            return;
         this.actor.disconnect(this._onEventId);
         this._onEventId = null;
     },