mpris: Cleanup _get_metadata method

* Update codestyle.
* Do not use exceptions.
* Most of the media properties cannot be None (length, track_number,
  play_count, title).
4 jobs for wip/jfelder/mpris-small-cleanups in 24 minutes and 40 seconds (queued for 1 second)
Status Job ID Name Coverage
  Check
passed #312019
flake8

00:01:16

 
  Build Flatpak
passed #312020
flatpak

00:22:18

 
  Deploy
passed #312021
review

00:00:26

passed #312022
allowed to fail manual
stop_review

00:00:38