remote_server: Handle data of non existent realms.

This commit adds code to make sure that the push
notification does not crash on receiving data for
a non-existent realm.

zerver/tests/test_push_notifications.py

@@ -1909,6 +1909,36 @@ class AnalyticsBouncerTest(BouncerTestCase):
             ],
         )
 
+    @override_settings(PUSH_NOTIFICATION_BOUNCER_URL="https://push.zulip.org.example.com")
+    @responses.activate
+    def test_non_existent_realm_uuid(self) -> None:
+        self.add_mock_response()
+        realm_info = get_realms_info_for_push_bouncer()
+
+        # Hard-delete a realm to test the non existent realm uuid case.
+        realm = Realm.objects.order_by("id").first()
+        assert realm is not None
+        deleted_realm_uuid = realm.uuid
+        realm.delete()
+
+        with mock.patch(
+            "zerver.lib.remote_server.get_realms_info_for_push_bouncer", return_value=realm_info
+        ) as m, self.assertLogs("zulip.analytics", level="WARNING") as analytics_logger:
+            send_server_data_to_push_bouncer(consider_usage_statistics=False)
+            m.assert_called()
+            realms = Realm.objects.all()
+            for realm in realms:
+                self.assertEqual(realm.push_notifications_enabled, True)
+                self.assertEqual(realm.push_notifications_enabled_end_timestamp, None)
+
+        self.assertEqual(
+            analytics_logger.output,
+            [
+                "WARNING:zulip.analytics:"
+                f"Received unexpected realm UUID from bouncer {deleted_realm_uuid}"
+            ],
+        )
+
 
 class PushNotificationTest(BouncerTestCase):
     @override