BUGFIX: return 1 when no MAC is passed to mac_exists()

If an interface doesn't have a MAC, mac_exists() is called with an
empty string that returns a list of all interfaces names.  Instead,
return 1 from the function so that the "network" line is not
generated.
Merge pull request #615 from aronparsons/skip-nic-with-no-mac

BUGFIX: return 1 when no MAC is passed to mac_exists()