Filter GCN notices lacking sky map info from creating sky maps

We need to prevent the creation of external sky maps when GCN notice contain no sky map information (effectively when all ra/dec/error are blank). There's a chance these cause an artificially high significance when overlapping a GW sky map and create a false alarm that would need to be retracted.

An example of this error is here: https://gracedb.ligo.org/events/E388215/view/.