Bug 23104: (bug 18925 follow-up): Fix null/empty behavior, again

Bug 23104: (bug 18925 follow-up): Fix null/empty behavior, again

Not kidding, it's again, and will certainly appear more.
The problem is that we should have a NULL value in DB to make things
more readable and logical. Now we need a quick fix.

Test plan:
Set "Current checkouts allowed" to unlimited (empty string)
Try to check an item out
=> Without this patch we are blocked with a "too many" message
=> With this patch applied the correct behavior is back!

Signed-off-by: Mark Tompsett <mtompset@hotmail.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
(cherry picked from commit 8c8e36cf7abf51113654c5e1c0497f918a1cd56c)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>