libtracker-extract: Initialize string as NULL
libtracker-extract: Initialize string as NULL

The pointer is used and shared later without any check. No know bug
about it and tests pass fine before and after.

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