BibKnowledge: /kb/export 500 error fix
BibKnowledge: /kb/export 500 error fix

* Fixes a bug where an HTTP 500 error was reported to the user trying
  to access /kb/export for a knowledge base whose kb_type was set to
  '' (i.e. empty string).

inveniosoftware/invenio,tiborsimko/invenio,inveniosoftware/invenio,tiborsimko/invenio