BUG 14693: Fixed vtkAssignAttribute for unnamed arrays.

vtkAssignAttribute was not handling unnamed arrays properly. Fixed that.
Also extended the test TestAssignAttribute to test this.

Change-Id: I0abc61dd9c14a63346669cd108a15013a26b131f
Merge topic '14693_assignattribute_and_unnamed_arrays' into master

49e0b6af BUG 14693: Fixed vtkAssignAttribute for unnamed arrays.