In the above changeset, we changed ordered entities to be edited according to
mandatory inlined constraints, though this leads to unpredictable order in
cases where there are no constraint. This leads to case where inlined relation
field may think there is no value related to a mandatory relation, while this
is simply because there is a value specified but to an entity that will be
created later (together with the relation).

To fix this, we've to use the fact that the RelationField.process_form_value
return None in cases where there is some entity to be created, while an empty
set when there are no linked entity. The new no_value() method allows to
differentiate between those different notion of 'no-value'.