Bug 6810: [QA Follow-up] Exit cronjob if pref not set

Bug 6810: [QA Follow-up] Exit cronjob if pref not set

As per suggestion of Robin on report 14840, it would be better to always
run the cronjob and only do something when the pref is set.
This patch adds a test in the cronjob and clears the former default of 14
days.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Removed the pref and ran the dbrev again: Fine.
Run the cronjob with -c -v -n: Prints exit warning.
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>