WebSearch: timestamp detection fix for empty sites
WebSearch: timestamp detection fix for empty sites

* Fixes get_database_last_updated_timestamp() so that it would not return
  None in case no records are present in the system.  This solves webcoll
  traceback issues when running on an empty site.

Reported-by: Ole Rønning <56d6cf41a1013b10c0a70ef7558478d852ff28bb@tindtechnologies.com>
Signed-off-by: Tibor Simko <c7116e45300b163846dc01f3ed02116e3f8ef7a5@cern.ch>

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