cubicweb #1251790 NOT NULL modifier not set on schema migration [waiting feedback]

When an attribute (or an inlined relation) is required, if the database supports it, a NOT NULL (sql) modifier is added on the corresponding column.

It works as expected when then entity type is added, but not when the attribute is added or updated during a migration (e.g. add_attribute())

NOTE: This was maybe done (or forgotten) on purpose since the constraint would be violated, for all existing entities, as soon as the column would be added. To bypass this, one would have to hack the fsschema object before calling add_attribute(), then restore the original rdef, then call sync_schema_prop_perms.

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