Bug 17669: [QA Follow-up] Allow zero in temp-uploads-days

Bug 17669: [QA Follow-up] Allow zero in temp-uploads-days

As requested by QA on comment33.
If the pref is 0 or the overriding command line parameter is 0, all
temporary files will be deleted. But if the pref is NULL or empty string,
we will not delete files.

Also adjusted the description of the preference in this regard.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>