simple_search_list_txn should return None, not 0.

[clokep]

This was noticed in #8127 (comment), fixing it separately since it was unrelated.

It looks like there's only a single caller to this:

synapse/synapse/storage/databases/main/__init__.py

Lines 562 to 578 in 30426c7

	async def search_users(self, term: str) -> Optional[List[Dict[str, Any]]]:
	"""Function to search users list for one or more users with
	the matched term.
	Args:
	term: search term
	Returns:
	A list of dictionaries or None.
	"""
	return await self.db_pool.simple_search_list(
	table="users",
	term=term,
	col="name",
	retcols=["name", "password_hash", "is_guest", "admin", "user_type"],
	desc="search_users",
	)

Which also gets called from a single spot:

synapse/synapse/rest/admin/users.py

Lines 596 to 616 in 97962ad

	async def on_GET(self, request, target_user_id):
	"""Get request to search user table for specific users according to
	search term.
	This needs user to have a administrator access in Synapse.
	"""
	await assert_requester_is_admin(self.auth, request)
	target_user = UserID.from_string(target_user_id)
	# To allow all users to get the users list
	# if not is_admin and target_user != auth_user:
	# raise AuthError(403, "You are not a server admin")
	if not self.hs.is_mine(target_user):
	raise SynapseError(400, "Can only users a local user")
	term = parse_string(request, "term", required=True)
	logger.info("term: %s ", term)
	ret = await self.handlers.store.search_users(term)
	return 200, ret

The comments around that endpoint seems to be lies (and I think that that endpoint is just completely broken?)