[IMP] fields: initialized computed fields to a null value instead of a failed one

This should ease compute methods that assign a list to "update" a
one2many/many2many field.  With a failed value, the update crashes.
[IMP] fields: infer required=True on related fields

One can infer related=True on a non-stored related field if all fields on the
path are related.  This cannot be done if the related field is stored: when you
create a record, the database row is created first, and the related field is
computed and stored afterwards.  Making the field required in that case would
trigger a non-null constraint violation.