Fix missing results on GND queries over all fields (with `q` param)

Deployed to staging:

http://api.lobid.org/subject?q=Informatik (issue: 0 results)
http://staging.api.lobid.org/subject?q=Informatik (fix: > 50 results)

Which is an extreme case of a general issue with all `q` queries against GND:

http://api.lobid.org/subject?q=Essen (issue: 4 results)
http://staging.api.lobid.org/subject?q=Essen (fix: > 50 results)

Thanks to @literarymachine for reporting the issue.