BUG: Fixed default filename generation in data bundle saving

When storage node existed but filename was NULL (e.g., when created an output node and run a CLI to set it) bundle saving crashed.
Now if thefilename is NULL then it is properly set and then restored to NULL. Restoring it to NULL is important because otherwise the node file would be saved by default into a temporary directory when the user opens the save dialog.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24623 3bd1e089-480b-0410-8dfb-8563597acbee