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 <ole@tindtechnologies.com>
Signed-off-by: Tibor Simko <tibor.simko@cern.ch>