send-bug-notifications OOPSes: IndexError: list index out of range

Since https://code.launchpad.net/~pelpsi/launchpad/+git/launchpad/+merge/442903 was deployed, we've been seeing lots of OOPSes like this (see `scripts.lp.internal::launchpad-production-logs/send-bug-notifications.log`):

  2023-06-14 00:01:20 ERROR Error while building email notifications.
  Traceback (most recent call last):
    File "/srv/launchpad.net/production/launchpad-rev-2edb915d3e46a38cd09521fa848436c630bd11ce/lib/lp/bugs/scripts/bugnotification.py", line 323, in get_email_notifications
      yield construct_email_notifications(batch)
    File "/srv/launchpad.net/production/launchpad-rev-2edb915d3e46a38cd09521fa848436c630bd11ce/lib/lp/bugs/scripts/bugnotification.py", line 206, in construct_email_notifications
      reason = data["sources"][0].reason_body
  IndexError: list index out of range
  2023-06-14 00:01:20 INFO OOPS-2840cc064513a0d4c93d870df417cfa7

I'm guessing that perhaps this happens when a user has every subscription via which they might receive a given bug muted, or something like that.