network: drop unneeded null-check for ip4 address

The pointer has already been dereferenced. Might have made more sense
to check if it's the empty string and treat that as error, but
nm_utils_ipaddr_valid also returns error on empty string so lets just
rely on the utility function doing the right thing here.

https://bugzilla.gnome.org/show_bug.cgi?id=708500