[migration] add_relation don't take into account `required=True`

How to reproduce this bug ?

take one of your cube, add poulet=String(required=True) in one class schema declaration, and run db-init. If you ask postgresql, you should find that poulet is NON-nullable ;
on an existing base, add poulet=String(required=True) in one class schema declaration, and add this attribute in the shell with add_attribute("MyClass", "poulet"). If you ask postgresql, you should find that poulet can be NULL, while it shouldn't.

Cubicweb version 3.28, with postgresql 11

Edited Nov 17, 2020 by François Ferry

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