fix: set a fallback for db user in connect()

In case we don't have a site name, `init()` won't be called, and we won't have a username

Signed-off-by: Akhil Narang <me@akhilnarang.dev>

frappe/__init__.py

@@ -291,7 +291,7 @@ def connect(
 	local.db = get_db(
 		host=local.conf.db_host,
 		port=local.conf.db_port,
-		user=local.conf.db_user,
+		user=local.conf.db_user or db_name,
 		password=None,
 	)
 	if set_admin_as_user: