m-si-audio-endpoint: do not use virtual media class for null sink nodes

The PulseAudio layer does not recognize virtual media classes.
6 jobs for ep-fix in 1 minute and 19 seconds (queued for 6 seconds)
Status Name Job ID Coverage
  Container
passed container_fedora #14952724

00:00:16

passed container_ubuntu #14952725

00:00:14

 
  Build
passed build_on_fedora_no_docs #14952727

00:00:35

passed build_on_fedora_with_docs #14952726

00:00:54

passed build_on_ubuntu_no_gir #14952729

00:00:51

passed build_on_ubuntu_with_gir #14952728

00:01:02