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)

Signed-off-by: Samuele Kaplun <samuele.kaplun@cern.ch>
So, what shall we do? IMHO we can happily not check anything in this specific upgrade recipe, since it can run multiple times.