playlists: Fix Tracker criticals

An ordering issue in _static_playlist_query_cb resulted in critical
warnings being emitted. get_string was always called, regardless of a
result being available and the async operation being finished.
Only call get_string after next_finish returns with a result.

Closes: #251
5 jobs for <span class="ref-name">wip/mschraal/tracker-critical-fixes</span> in 11 minutes and 31 seconds (queued for 1 second)
Status Job ID Name Coverage
  Check
passed #165972
flake8

00:02:45

 
  Build Flatpak
passed #165973
flatpak

00:07:37

 
  Deploy
passed #166093
review

00:00:42

passed #165975
allowed to fail manual
stop_review

00:00:26

failed #165974
review

01:01:28