Fix PUT /api/v1/database-connections/<conn_id> request

[b-tsao]

The update database-connections api was broken by this PR 31b88df after adding support for schemas

Where the response to the call would be

{"error_code":"invalid_database_connection","message":"'list' object has no attribute 'items'","description":null,"detail":{"alias":"SomeAlias","use_ssh":false,"connection_uri":"gAAAAABmcios2gjJ4CI9-D7jd5vjIuA1fqzC3HGGBQOlxKn4-TGdKSH6X4nmy0onQgJ9WNWJlp79LHwzE0Che12aQ2T7eiV2q3XgI6g4KAc46FBG9keA0SyO-PLpRc4Ewa1y39sSfqCq4S_T05OF_ZVNVWqpHpVJ3rIXQLFsRKyB-YThpJKCyws=","schemas":null,"path_to_credentials_file":null,"llm_api_key":null,"ssh_settings":null,"file_storage":null}}