si-audio-adapter: parse the rate optionally

We can use spa_format_audio_raw_parse() because we need to parse
the samplerate here optionally.

Many virtual sinks will not set a rate, which makes parsing fail and
the node will end up with the default channels/positions, which is
most likely wrong.

Fixes channel positions for virtual sinks.
6 jobs for parse-rate-optional in 1 minute and 47 seconds
Status Name Job ID Coverage
  Container
passed container_fedora #14690666

00:00:32

passed container_ubuntu #14690667

00:00:37

 
  Build
passed build_on_fedora_no_docs #14690669

00:01:03

passed build_on_fedora_with_docs #14690668

00:01:09

passed build_on_ubuntu_no_gir #14690671

00:01:03

passed build_on_ubuntu_with_gir #14690670

00:01:05