WindowManager: Allow $returnFocusTo to be null
WindowManager: Allow $returnFocusTo to be null

Bug: T150556
Change-Id: Iaa7dffc13fdb1057d0a003b21812ba163c7cff05

wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui