message_send: Add support to send message with an 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.

Adds a `empty_topic_name` client capability to allow client
to specify whether it supports empty string as a topic name.

Adds backward compatibility for:
- `subject` field in the `message` event type

zerver/lib/home.py

@@ -157,6 +157,7 @@ def build_page_params_for_home_page_load(
         user_list_incomplete=True,
         include_deactivated_groups=True,
         archived_channels=True,
+        empty_topic_name=True,
     )
 
     if user_profile is not None: