streams: Handle empty topic only streams being used for announcements.

This commit updates code to send messages to "general chat" topic
if streams used for announcements for "New user signups",
"New created streams", "Moderation requests" and "Zulip updates"
have topics policy set to allow only "general chat" messages.

zerver/tests/test_subs.py

@@ -30,6 +30,7 @@ from zerver.actions.streams import (
     deactivated_streams_by_old_name,
     do_change_stream_group_based_setting,
     do_deactivate_stream,
+    do_set_stream_property,
     do_unarchive_stream,
 )
 from zerver.actions.user_groups import bulk_add_members_to_user_groups, check_add_user_group
@@ -3431,6 +3432,7 @@ class SubscriptionAPITest(ZulipTestCase):
         self.assertEqual(msg.recipient.type, Recipient.STREAM)
         self.assertEqual(msg.recipient.type_id, new_stream_announcements_stream.id)
         self.assertEqual(msg.sender_id, self.notification_bot(self.test_realm).id)
+        self.assertEqual(msg.topic_name(), "new channels")
         expected_msg = f"@_**{invitee_full_name}|{invitee.id}** created a new channel #**{invite_streams[0]}**."
         self.assertEqual(msg.content, expected_msg)
 
@@ -3444,6 +3446,41 @@ class SubscriptionAPITest(ZulipTestCase):
         )
         self.assertEqual(msg.content, expected_msg)
 
+    def test_sucessful_subscription_notifies_in_empty_topic_only_stream(self) -> None:
+        invitee = self.example_user("iago")
+        invitee_full_name = "Iago"
+
+        current_stream = self.get_streams(invitee)[0]
+        invite_streams = self.make_random_stream_names([current_stream])[:1]
+
+        new_stream_announcements_stream = get_stream(current_stream, self.test_realm)
+        self.test_realm.new_stream_announcements_stream_id = new_stream_announcements_stream.id
+        self.test_realm.save()
+
+        do_set_stream_property(
+            new_stream_announcements_stream,
+            "topics_policy",
+            StreamTopicsPolicyEnum.empty_topic_only.value,
+            invitee,
+        )
+
+        self.subscribe_via_post(
+            invitee,
+            invite_streams,
+            extra_post_data=dict(
+                announce="true",
+                principals=orjson.dumps([self.user_profile.id]).decode(),
+            ),
+        )
+
+        msg = self.get_second_to_last_message()
+        self.assertEqual(msg.recipient.type, Recipient.STREAM)
+        self.assertEqual(msg.recipient.type_id, new_stream_announcements_stream.id)
+        self.assertEqual(msg.sender_id, self.notification_bot(self.test_realm).id)
+        self.assertEqual(msg.topic_name(), "")
+        expected_msg = f"@_**{invitee_full_name}|{invitee.id}** created a new channel #**{invite_streams[0]}**."
+        self.assertEqual(msg.content, expected_msg)
+
     def test_successful_cross_realm_notification(self) -> None:
         """
         Calling POST /json/users/me/subscriptions in a new realm