typing: Add support for empty topic name.

This commit is a part of the work to support empty string
as a topic name.

Previously, empty string was not a valid topic name.

Now, typing operation supports empty topic name.

Adds backward compatibility for:
-  `topic` field in the `typing` event type

zerver/tornado/event_queue.py

@@ -1642,6 +1642,27 @@ def process_user_topic_event(event: Mapping[str, Any], users: Iterable[int]) ->
                 client.add_event(empty_topic_name_fallback_event)
 
 
+def process_stream_typing_notification_event(
+    event: Mapping[str, Any], users: Iterable[int]
+) -> None:
+    empty_topic_name_fallback_event: Mapping[str, Any] | dict[str, Any]
+    if event.get("topic") == "":
+        empty_topic_name_fallback_event = dict(event)
+        empty_topic_name_fallback_event["topic"] = Message.EMPTY_TOPIC_FALLBACK_NAME
+    else:
+        empty_topic_name_fallback_event = event
+
+    for user_profile_id in users:
+        for client in get_client_descriptors_for_user(user_profile_id):
+            if not client.accepts_event(event):
+                continue
+
+            if client.empty_topic_name:
+                client.add_event(event)
+            else:
+                client.add_event(empty_topic_name_fallback_event)
+
+
 def process_notification(notice: Mapping[str, Any]) -> None:
     event: Mapping[str, Any] = notice["event"]
     users: list[int] | list[Mapping[str, Any]] = notice["users"]
@@ -1672,6 +1693,8 @@ def process_notification(notice: Mapping[str, Any]) -> None:
         process_user_group_name_update_event(event, cast(list[int], users))
     elif event["type"] == "user_topic":
         process_user_topic_event(event, cast(list[int], users))
+    elif event["type"] == "typing" and event["message_type"] == "stream":
+        process_stream_typing_notification_event(event, cast(list[int], users))
     elif event["type"] == "cleanup_queue":
         # cleanup_event_queue may generate this event to forward cleanup
         # requests to the right shard.