audio-endpoint: configure adapter for null sink with monitor.channel-volumes

This allows the channelVolumes and mute props to affect the output
of the null sink's monitor ports. This way we will be able to use
channelVolumes to track the configured volume of the endpoint
and the monitorVolumes to implement volume ducking.

modules/module-si-audio-endpoint.c

@@ -225,6 +225,7 @@ si_audio_endpoint_enable_active (WpSessionItem *si, WpTransition *transition)
           PW_KEY_FACTORY_NAME, "support.null-audio-sink",
           PW_KEY_NODE_DESCRIPTION, desc,
           SPA_KEY_AUDIO_POSITION, "FL,FR",
+          "monitor.channel-volumes", "true",
           NULL));
   if (!self->node) {
     wp_transition_return_error (transition,