BibRank: fix empty citation data cacher
BibRank: fix empty citation data cacher

* In case no citation data is available in the rnkMETHOD table,
  the timestamp_verifier should not throw an error but return
  '0000-00-00 00:00:00'.

inveniosoftware/invenio,tiborsimko/invenio,tiborsimko/invenio,inveniosoftware/invenio