Mark basis set as invalid if no atoms found.
Mark basis set as invalid if no atoms found.

Ref discussion in:
http://sourceforge.net/mailarchive/forum.php?thread_name=510E81A7.3080406%40pitt.edu&forum_name=avogadro-discuss

Change-Id: I685f000f4a5bd8a05ee6ded508cb30b78a17c1d0

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