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>
Is it necessary at all? I have added this by learning by example from other upgrade recipe. It the condition it too strict it will not match, and therefore the upgrade will be run. This upgrade happens to be idempotent, so we can even remove the `if` completely...