BibIndex: tag.recjsonvalue NOT NULL

* Complements upgrade recipe invenio_2014_08_13_tag_recjsonvalue by
  adding the further property NOT NULL to the tag.recjsonvalue column
  definition.
  (closes #1947) (PR #2259)

Signed-off-by: Samuele Kaplun <samuele.kaplun@cern.ch>
Reviewed-by: Tibor Simko <tibor.simko@cern.ch>
Reviewed-by: Jiri Kuncar <jiri.kuncar@cern.ch>