FileManager+LibGUI: Allow drop on entire DirectoryView

Previously, drag and drop would only work when dragging between node
items on a DirectoryView. This commit makes it possible to drag files to
the empty area of the DirectoryView and copy files more easily between
windows.

Libraries/LibGUI/AbstractView.cpp

@@ -350,9 +350,6 @@ void AbstractView::drop_event(DropEvent& event)
         return;
 
     auto index = index_at_event_position(event.position());
-    if (!index.is_valid())
-        return;
-
     if (on_drop)
         on_drop(index, event);
 }