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

tools/linter_lib/custom_check.py

@@ -37,6 +37,7 @@ FILES_WITH_LEGACY_SUBJECT = {
     # This has lots of query data embedded, so it's hard
     # to fix everything until we migrate the DB to "topic".
     "zerver/tests/test_message_fetch.py",
+    "zerver/tests/test_message_topics.py",
 }
 
 shebang_rules: list["Rule"] = [