Prefer positives joint FARs over negative joint FARs when selecting preferred external event

Looking at the logs for https://gracedb.ligo.org/superevents/S250223dk/view/ we should have switched to using the spatial joint FAR for https://gracedb.ligo.org/events/E557177/view/ after the sky map came out. I suspect based on the logs the following events occurred:

1. E557001 was preferred since it was the only event originally.
2. The timing info for E557177 came in and updated the superevent, due the fact that E557001 had a NOT_GRB label while E557177 didn't.
3. The spatial info for E557177 came in but this does not update the superevent when it should have. This seems to have occurred because the negative FARs were preferred over the postive ones (labels and search were identical, so the tie breaker is now the FARs).

The solution for this is to add logic to set negative FARs to -float('inf') like we currently do for Nones:

https://git.ligo.org/emfollow/gwcelery/-/blob/019720058458090cfac8991791152dd8d4e92aa0/gwcelery/tasks/raven.py#L427-431