Block EM_COINC labels for SubGRBTargeted before a sky map is available; fixes issue in !1461

This MR fixes an issue in !1461 (merged) where although we block EM_COINC for search='SubGRBTargeted' events when NOT_GRB is present, this label isn't applied in the initial IGWN alert (see https://chat.ligo.org/ligo/pl/1xa1ej9g47fgipkgjswgzfrboe). This means this initial IGWN alert will apply the EM_COINC, making the changes in !1461 (merged) useless.

This MR fixes this by requiring there also to be a joint sky map calculation to be available (something never available with the first IGWN alert). Therefore if we block this first IGWN alert due to not having sky maps yet and then block whenever NOT_GRB is present, this should prevent this label from being applied for NOT_GRB SubGRBTargeted events.