Python3 branches segfault when None is given for 's' format in return message.

[hmvp]

This gives strange results:

try:
    connection.send_method_return(message, 's', [None])
    logger.error('x')
except Exception as e:
    # Format might be wrong with the arguments (often when an argument is None)
    logger.error('s')

    logger.exception(e)
try:
    connection.send_method_return(message, 's', [None])
    logger.error('x2')
except Exception as e:
    logger.error('s2')
    logger.exception(e)