don't print a warning about empty nodes in hoomd xml file, if node has the attribute num="0" set.
don't print a warning about empty nodes in hoomd xml file,
if node has the attribute num="0" set.


git-svn-id: 9be72bd8b79bf6944680671de68cae08993c9086@1960 fa922fa7-2fde-0310-acd8-f43f465a7996

joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue