BUG: SliceViewAnnotations: Do not show blank when corner annotation is off.

Fixes 0004043

Note that while setting the text of the corner annotation to a space
avoid the blank area from showing up, this doesn't address the root cause.
There is probably something wrong in either ctkVTKSliceView or VTK itself.

The problem is still reproducible running the following snippet in the
Slicer python interactor:

v = ctk.ctkVTKSliceView()
v.show()
v.cornerAnnotationText = "Hello"
v.forceRender()
v.cornerAnnotationText = ""
v.forceRender()

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24649 3bd1e089-480b-0410-8dfb-8563597acbee