pulseaudio: let PA choose default device

PA default device is selected by passing nullptr to
pa_stream_connect_playback and pa_stream_connect_record,
which is the way described in the official documentation:
https://freedesktop.org/software/pulseaudio/doxygen/stream_8h.html#ab9544f6677af133fbe81bf8a21eb489c



We used to attempt to guess the default device by incorrectly using
information from the PA server (pa_context_get_server_info).

Change-Id: Ifa8109b0ba86b223a2eca3ff136822b338fbf730
Reviewed-by: Guillaume Roguez <guillaume.roguez@savoirfairelinux.com>