BUG: Update SliceViewAnnotation to avoid error when restoring scene view

This commit avoid the test "py_RSNA2012ProstateDemo" to produce the errors
reported below.

Notes:

(1) this commit does NOT fix the underlying issue. Indeed, after
restoring a scene view, no annotation are displayed.

(2) after restoring the "" scene view, there is still error related to
missing active camera:

   WorldToView: no active camera, cannot compute world to view, returning 0,0,0

(3) after the test complete, when Slicer shutdown, the following warnings
are still reported:

   Warning: In /path/to/VTKv6/Common/ExecutionModel/vtkAlgorithm.cxx, line 1421
   vtkOpenGLImageMapper (0x18a48f50): Attempt to get connection index 0 for input port 0, which has 0 connections.

The associated error messages were:

// ------------
Traceback (most recent call last):
File "/path/to/Slicer-build/lib/Slicer-4.4/qt-scripted-modules/DataProbeLib/SliceViewAnnotations.py", line 622, in updateCornerAnnotations
self.makeAnnotationText(caller)
File "/path/to/Slicer-build/lib/Slicer-4.4/qt-scripted-modules/DataProbeLib/SliceViewAnnotations.py", line 808, in makeAnnotationText
sliceViewName = sliceNode.GetLayoutName()
AttributeError: 'NoneType' object has no attribute 'GetLayoutName'
// ------------

and

// ------------
Restoring scene view Preoperative T2w MRI ...
test_RSNA2012ProstateDemo (RSNA2012ProstateDemo.RSNA2012ProstateDemoTest) ... Traceback (most recent call last):
File "/path/to/Slicer-build/lib/Slicer-4.4/qt-scripted-modules/DataProbeLib/SliceViewAnnotations.py", line 605, in updateCornerAnnotations
self.makeAnnotationText(caller)
File "/path/to/Slicer-build/lib/Slicer-4.4/qt-scripted-modules/DataProbeLib/SliceViewAnnotations.py", line 808, in makeAnnotationText
foregroundOpacity = sliceCompositeNode.GetForegroundOpacity()
AttributeError: 'NoneType' object has no attribute 'GetForegroundOpacity'
// ------------

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24127 3bd1e089-480b-0410-8dfb-8563597acbee