pulse-server: handle 0 sink/source name or index as default

Some apps use 0 to get some sort of sink/source info. PulseAudio
likely has a source/sink at index 0 but PipeWire certainly does not.

Asking for 0 source/sink is always because of some hardcoded values
in the app and we can return the default source/sink without causing
any problems for other apps.

See #756