return empty path for untitled string instead of /path/to/untitled.mb

I *think* this is a bug, but currently pm.sceneName() on an untitled scene returns Path('/path/to/untitled.mb') instead of Path(''). This is because _mel.eval('untitledFilename()') returns u'untitled', with no extension, so the comparison fails.

Here's a patch that fixes this.

cheers,
-Mark