Trebuchet : Check if target is null for cancelled events

If a drag is cancelled on the workspace, ensure not to reference
it in onDropCompleted since it will be null as per what DragController
passes in.

Reproduction steps :

Long press a hotseat icon
While dragging it, turn off screen.

Change-Id: If587e7c9c37ea2e24b7f9cc85db8249ab34dd19f