Pure-Python connector sends an empty user name when user is not set

In 2.0, with the pure-Python implementation, mariadb.connect() without a user argument sends an empty user name in the handshake, so the server rejects the connection:

mariadb.OperationalError: Access denied for user ''@'127.0.0.1' (using password: NO) (errno: 1045, sqlstate: 28000)

The C extension falls back to the OS login name instead, so the same call authenticates as the current user and works with unix_socket authentication.