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 a8633a5812)
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
(cherry picked from commit 5edd76121b)
Signed-off-by: Pedro Amorim <pedro.amorim@ptfs-europe.com>

api/v1/swagger/definitions/ill_request.yaml

@@ -24,7 +24,9 @@ properties:
     type: string
     description: Internal library identifier
   requested_date:
-    type: string
+    type:
+      - string
+      - "null"
     format: date
     description: Date the request was placed by the patron
   replied_date: