If draining was not requested, generate silence instead if there is no
data in the ring buffer.

The problem is that pw_stream_flush() appears to disable the stream
permanently, even though there is no state_changed callback - the
stream state remains at PW_STREAM_STATE_STREAMING, but the stream is
defunct.  I have no idea why and I havn't found any documentation
about it.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1219