Bug 34130: Make requested_date nullable in api definition

Bug 34130: Make requested_date nullable in api definition

This patch allows the requested_date for an ILL request to be NULL to accomodate
older data

To test:
1 - Install the Koha 2 Koha ILL plugin:
    https://gitlab.com/koha-community/plugins/koha-plugin-ill-koha
2 - Enable the ILL  system preference
3 - Force an ILL request with minimal data from backend:
    INSERT INTO illrequests (borrowernumber,biblio_id,branchcode,backend,status) VALUES (5,3,'CPL','Koha','placed');
4 - View the ILL table
5 - Error:
    {"message":"Expected string - got null.","path":"\/body\/0\/requested_date"}
6 - Apply patch
7 - Table loads successfully

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
(cherry picked from commit a8633a58129bc2bb6b26986973501aa6649f2c78)
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>