Fix crashes when trying to send blacklist emails

Created by: cburchert

Fix #391 (closed). We did not correctly handle that the end_time of an event is nullable. To fix:

1. Patching a blacklist entry to remove the end_time now doesn't schedule an email about the entry timing out anymore.
2. When the scheduler runs an event to send an email and the end_time was deleted in the meantime we return instead of crashing.