Bug 29877: Make POST /holds handle maxreserves correctly

Bug 29877: Make POST /holds handle maxreserves correctly

The current implementation doesn't consider the following values for the
syspref: undef and 0.

The tests mistakenly didn't cover them.

To test:
1. Apply the regression tests patch
2. Run:
   $ kshell
  k$ prove t/db_dependent/api/v1/holds.t
=> FAIL: Tests fail, obvious warnings about comparing undefined values
         too.
3. Apply this patch
4. Repeat 2
=> SUCCESS: Tests pass!
5. You can try on Postman as well
=> SUCCESS: Behavior is correct!
6. Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Andrew Fuerste-Henry <andrew@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>