Bugfix: crash on highstate population when empty path is passed

There are situations when minion does not likes module sync'ing and simply crashes the ``highstate`` procedure. Because of returned file list includes one empty element, this happens precisely in ``saltutils.py``, here: https://github.com/saltstack/salt/blob/develop/salt/modules/saltutil.py#L109

The empty element is happening due to passed URL ``salt://top.sls`` during modules sync'ing here: https://github.com/saltstack/salt/blob/develop/salt/fileclient.py#L510 and this causes to return no path here: https://github.com/saltstack/salt/blob/develop/salt/fileclient.py#L518 which renders to one empty element for ``saltutil.py`` part.

Hope this helps.