Bug 28248: aqorders.created_by is nullable

Bug 28248: aqorders.created_by is nullable

This patch introduces a change to the spec to acknowledge that the field
is nullable. This is fixed in 21.05+ as part of bug 20212.

To test:
1. Have an order with created_by = NULL
2. Use your favourite REST tool (Advanced REST Client(libre), Postman?) to fetch the order
=> FAIL: It throws a nasty error about non-nullable field being null
3. Apply this patch
4. Reload all
5. Repeat 2
=> SUCCESS: It returns things, created_by is NULL, as it should
6. Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Victor Grousset/tuxayo <victor@tuxayo.net>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>