fix issue reported in '[#33876] bpy.path.ensure_ext adds extension twice / extra period if filename empty, just a period or equal to extension'

For python operators that used the ExportHelper mix-in class, an empty file field would become '.ext', entering and existing the text field would become '.ext.ext',
Now only add an extension if the filename part of the path is set, so '.ext' will still become '.ext.ext' but having only the extension isn't so likely to happen in the first place now.

This is a different fix then the changes suggested in the report but I'd prefer to keep path functions stupid+predictable.


git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@53803 954f8c5b-7b00-dc11-b283-0030488c597c