typing_notification: Notify server only if topic already exists.

Sending this request wouldn't be of much use as we are
displaying notifications in a topic narrow for the recieving users.
They cannot be in a topic narrow that doesn't exist already for
them to benefit from these typing notifications.

static/js/typing.js

@@ -7,6 +7,7 @@ import * as compose_pm_pill from "./compose_pm_pill";
 import * as compose_state from "./compose_state";
 import * as people from "./people";
 import * as stream_data from "./stream_data";
+import * as stream_topic_history from "./stream_topic_history";
 
 // This module handles the outbound side of typing indicators.
 // We detect changes in the compose box and notify the server
@@ -91,6 +92,9 @@ export function get_recipient() {
         const stream_name = compose_state.stream_name();
         const stream_id = stream_data.get_stream_id(stream_name);
         const topic = compose_state.topic();
+        if (!stream_topic_history.get_recent_topic_names(stream_id).includes(topic)) {
+            return null; // Send notification only if topic already exists.
+        }
         return {stream_id, topic};
     }
     return null;