Bug 22739: Test SelfCheckInTimeout <> 0, not just defined

Bug 22739: Test SelfCheckInTimeout <> 0, not just defined

On sci-main.pl the preference value is tested for 'defined'. This patch
tests it is different than empty string too. This was causing a
JavaScript syntax error, that made it display a wrong message.

To test:
1 - Blank the syspref SelfCheckInTimeout
2 - Attempt to checkin something via sci
=> FAIL: The 'Your request included no check-ins.' message is printed.
3 - Apply this patch, reload the sci-main.pl page
4 - Attempt to checkin something via sci
=> SUCCESS: The UI correctly stacks the scanned barcodes
5 - Sign off :-D

Signed-off-by: Liz Rea <wizzyrea@gmail.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>