BUG: Fixed node selector node type specification syntax

There was a common syntax error in defining single-element Qt string lists in Python.

The correct way of defining a single element list:
self.outputSelector.nodeTypes = ["vtkMRMLScalarVolumeNode"]

The correct way of defining a single element tuple:
self.outputSelector.nodeTypes = "vtkMRMLScalarVolumeNode",

Currently many modules try to define a single-element tuple like this:
self.outputSelector.nodeTypes = ( ("vtkMRMLScalarVolumeNode"), "" ) => this is long and ugly and also incorrect, as the tuple has actually two elements, "vtkMRMLScalarVolumeNode" and an empty element; the empty element generates the dysfunctional "Create new node" and "Create new node as..." options.

The syntax error is fixed in all modules and a check was added to ignore empty elements in the node type list (in case the syntax error occurs again in the future).

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24409 3bd1e089-480b-0410-8dfb-8563597acbee