Rerun RAVEN pipeline when `NOT_GRB` is removed only if a coincidence with `EM_COINC`

There was an error with 5e90d979 where we would rerun the RAVEN alert pipeline when the label NOT_GRB was removed from a GRB candidate. However, we should only rerun this when there is a known coincidence marked by the EM_COINC label. This error has been captured here:

https://sentry.io/organizations/ligo-caltech/issues/3694742313/?query=is%3Aunresolved+Nonetype&referrer=issue-stream&statsPeriod=14d

This could be fixed by adding an additional condition here:

https://git.ligo.org/emfollow/gwcelery/-/blob/7a6c3ef174e3eb88edc3368adbcf994a9f96bba2/gwcelery/tasks/external_triggers.py#L336