send_email: Delete ScheduledEmail objects with no recipients.

9d97af6ebb addressed the one major source of inconsistent data which
would be solved by simply re-attempting the ScheduledEmail row.  Every
other instance that we have seen since then has been a corrupt or
modified database in some way, which does not self-resolve.  This
results in an endless stream of emails to the administrator, and no
forward progress.

Drop this to a warning, and make it remove the offending row.  This
ensures we make forward progress.

zerver/lib/send_email.py

@@ -477,10 +477,14 @@ def deliver_scheduled_emails(email: ScheduledEmail) -> None:
     data = orjson.loads(email.data)
     user_ids = list(email.users.values_list("id", flat=True))
     if not user_ids and not email.address:
-        # This state doesn't make sense, so something must be mutating,
-        # or in the process of deleting, the object. We assume it will bring
-        # things to a correct state, and we just do nothing except logging this event.
-        logger.error("ScheduledEmail id %s has empty users and address attributes.", email.id)
+        # This state doesn't make sense, so something must have mutated the object
+        logger.warning(
+            "ScheduledEmail %s at %s had empty users and address attributes: %r",
+            email.id,
+            email.scheduled_timestamp,
+            data,
+        )
+        email.delete()
         return
 
     if user_ids:

zerver/tests/test_users.py

@@ -1698,16 +1698,21 @@ class ActivateTest(ZulipTestCase):
         assert email is not None
         email.users.remove(*to_user_ids)
 
+        email_id = email.id
+        scheduled_at = email.scheduled_timestamp
         with self.assertLogs("zulip.send_email", level="INFO") as info_log:
             deliver_scheduled_emails(email)
         from django.core.mail import outbox
 
         self.assert_length(outbox, 0)
-        self.assertEqual(ScheduledEmail.objects.count(), 1)
+        self.assertEqual(ScheduledEmail.objects.count(), 0)
         self.assertEqual(
             info_log.output,
             [
-                f"ERROR:zulip.send_email:ScheduledEmail id {email.id} has empty users and address attributes."
+                f"WARNING:zulip.send_email:ScheduledEmail {email_id} at {scheduled_at} "
+                "had empty users and address attributes: "
+                "{'template_prefix': 'zerver/emails/followup_day1', 'from_name': None, "
+                "'from_address': None, 'language': None, 'context': {}}"
             ],
         )