BUG: Fixed crash caused in Model display node widget

Slicer crashed when updated a filter pipeline that generated a polydata that was used by the model node.

There was an infinite loop (and stack overflow) caused by automatically selecting the first scalar attribute when the polydata is regenerated by the pipeline.

Fixed by allowing having 'none' selected as displayed scalar in the model widget and therefore avoiding automatic scalar selection when scalars are added to the data set and so avoiding infinite loop.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24502 3bd1e089-480b-0410-8dfb-8563597acbee