fetch_messages: Add support for empty topic names.

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 `allow_empty_topic_name` boolean parameter to `GET /messages`
and `GET /messages/{message_id}` endpoints to decide whether the
topic names in the fetched messages can be empty strings.

If False, the topic names in the fetched message will have the
value of `realm_empty_topic_display_name` field in `POST /register`
response replacing "" for channel messages.

tools/linter_lib/custom_check.py

@@ -248,6 +248,11 @@ python_rules = RuleList(
             "exclude_line": {
                 ("zerver/lib/message.py", "message__subject__iexact=message.topic_name(),"),
                 ("zerver/views/streams.py", "message__subject__iexact=topic_name,"),
+                ("zerver/lib/message_cache.py", 'and obj["subject"] == ""'),
+                (
+                    "zerver/lib/message_cache.py",
+                    'obj["subject"] = Message.EMPTY_TOPIC_FALLBACK_NAME',
+                ),
             },
             "include_only": {
                 "zerver/data_import/",