Bug 10843: fix crash that can occur when confirming hold if ReservesMaxPickUpDelay...

Bug 10843: fix crash that can occur when confirming hold if ReservesMaxPickUpDelay is undefined

Test plan:
1) set an empty string for the ReservesMaxPickUpDelay pref
2) place a hold on an item
3) check in the item
4) click on "Print and confirm"
5) an error occurs
> The 'days' parameter (undef) to DateTime::Duration::new was an 'undef'
6) apply the patch
7) repeat steps 1 to 4
8) the error does not occur anymore.

Signed-off-by: Srdjan <srdjan@catalyst.net.nz>
Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
An empty string didn't do it for me, I had to set the
variable for the systempreference to NULL. I am not sure
if this can happen when editing from the interface, but
this change should not have any ill side effects and it has
unit tests!

Signed-off-by: Galen Charlton <gmc@esilibrary.com>