libtracker-data: Fix gvdb lookup for not existing ontology resources
libtracker-data: Fix gvdb lookup for not existing ontology resources

Fixes GB#638159.

outofbits/tracker,outofbits/tracker,hoheinzollern/tracker,hoheinzollern/tracker,hoheinzollern/tracker,hoheinzollern/tracker,outofbits/tracker,outofbits/tracker,hoheinzollern/tracker,outofbits/tracker,hoheinzollern/tracker,outofbits/tracker,hoheinzollern/tracker,outofbits/tracker