BibFormat: fix for fulltext file format

* Avoids exception for records containing only additional files
  and no main file.

Reviewed-by: Tibor Simko <tibor.simko@cern.ch>
Merge branch 'maint-1.1'

* maint-1.1:
  BibFormat: fix for fulltext file format
Merge branch 'master' into next

* master:
  BibFormat: fix for fulltext file format