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>
Depending on escaping configuration testing for the whole string with "`" might be tricky. Can't we find other workaround?