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
2 jobs for master in 6 minutes and 33 seconds (queued for 1 second)
Status Job ID Name Coverage
  Check
passed #166383
flake8

00:01:30

 
  Build Flatpak
passed #166384
flatpak

00:05:03