BUG: avoid a warning dialog in Editor when scene is closed

The warning message was triggered because the validity of the
volume state is checked when nodes are added or removed from the
scene, but it doesn't make sense to warn about the missing master
volume if there is also no merge volume.

Thanks to Alexandra Prior for pointing this out.

http://slicer-users.65878.n3.nabble.com/Editor-td4027182.html

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22858 3bd1e089-480b-0410-8dfb-8563597acbee