BibField: correct 'schema' usage
BibField: remove None values from rec_json
BibField: fix for missing producer rule

* Changes MARC producer to catch and log errors when no producer rule
  exists for a given field.
BibField: fix utf8 encoding problem
BibField: better error handling when parsing rules

Co-authored-by: Lars Holm Nielsen <lars.holm.nielsen@cern.ch>>