BUG: don't try updating a 2D displayable manager if there is no slice node

This is an invalid condition that can lead to errors being generated
by the rest of the displayable manager code. The condition that this
checks for happens when a slice view is being created but has not
yet been fully configured (that is, this can be triggered when the
scene is assigned to the slice view, but the slice node has not
yet been assigned; in this case the slice node being assigned will
trigger the needed update).

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23325 3bd1e089-480b-0410-8dfb-8563597acbee