COMP: Fix "pointer-bool-conversion" warnings in VolumeHeaderlessStorageNode

FileDimensions and FileSpacing are not dynamically allocated, so
these pointers to arrays are always true.

Slicer/Libs/MRML/Core/vtkMRMLVolumeHeaderlessStorageNode.cxx:152:13: warning: address of array 'this->FileDimensions' will always
evaluate to 'true' [-Wpointer-bool-conversion]
if (this->FileDimensions)
~~ ~~^~~~~~
Slicer/Libs/MRML/Core/vtkMRMLVolumeHeaderlessStorageNode.cxx:158:13: warning: address of array 'this->FileSpacing' will always
evaluate to 'true' [-Wpointer-bool-conversion]
if (this->FileSpacing)
~~ ~~^~~

From: Hans Johnson <hans-johnson@uiowa.edu>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24619 3bd1e089-480b-0410-8dfb-8563597acbee