jsonalchemy: hotfix for optional fields

• Fixes an unexpected behaviour which happen when a field in the conversion sheet is not present in the marcxml (e.g. email). When marcxml is transformed to dict everything is fine, but when it is converted back to marcxml, it will get into an infinite loop because the field is not present in the dict. Hence it will try to eval the field and it will end in an infinite import loop.

Signed-off-by: Guillaume Lastecoueres <guillaume.lastecoueres@cern.ch>
Reviewed-by: Jiri Kuncar <jiri.kuncar@cern.ch>