Bug 19230: Preventing warn when deleting course

Bug 19230: Preventing warn when deleting course

To test:
1) Ensure UseCourseReserves is enabled
2) Go to Course Reserves, create a course
3) When you are taken to the course page, you should notice the
'Argument "" isn't numeric in numeric gt' warn
4) Delete course
5) Notice warn, and that no confirm message popped up
6) Apply patch
7) Create a course again. Confirm the 'Argument "" isn't numeric in
numeric gt' warn did not show
8) Delete the course
9) Notice no warn and the confirm message pops up

Sponsored-by: Catalyst IT
Signed-off-by: Roch D'Amour <roch.damour@inlibro.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>