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).
Merge branch 'maint-1.0' into maint-1.1

* maint-1.0:
  BibKnowledge: /kb/export 500 error fix

Conflicts:
	modules/bibknowledge/lib/bibknowledgeadmin.py