Keep the DHCP generated resolv.conf when there are no interfaces configured.

The commit that fixed issue 7293 inadvertantly broke an implicit workflow.  If you depend
on FreeNAS' "feature" of doing DHCP on every interface when you don't have any interfaces
configured then the fix for 7293 will cause the resolv.conf created by dhclient to get
overwritten with an empty resolv.conf generated by the GUI.

The change restores the implicit behavior and maintains the fix for 7293.

Ticket: #7335