Enable nullability in Content.Client (#3257)

* Enable nullability in Content.Client

* Remove #nullable enable

* Merge fixes

* Remove Debug.Assert

* Merge fixes

* Fix build

* Fix build

Content.Client/Utility/DragDropHelper.cs

@@ -45,7 +45,7 @@ namespace Content.Client.Utility
         /// <summary>
         /// Current thing being dragged or which mouse button is being held down on.
         /// </summary>
-        public T Dragged { get; private set; }
+        public T? Dragged { get; private set; }
 
         // screen pos where the mouse down began for the drag
         private Vector2 _mouseDownScreenPos;