Stream is now able to handle input_device = None

As written in the doc string:

"If no input or output device (or True) is specified, the
        default input/output device is taken. For input/output-only
        streams, proved None or False as input/output-device."

If there is no input_device or output_device, simply ignore it and don't create the structures needed for PortAudio. My patch provides the necessery changes (although only tested with input_device = None). Additionally I've updated the callback which now handles disabled input / output devices.