m-mixer-api: make sure the enum param iterator is valid

Some audio nodes such as null-audio-sink nodes don't support Props or Route enum
params.
2 jobs for mixer-fix in 1 minute and 14 seconds
Status Job ID Name Coverage
  Container
passed container_fedora #9913050

00:00:24

 
  Build
passed build_on_fedora #9913051

00:00:50