cubicweb-seda #12515331 ensure user pick at least one element of a choice [validation pending]

due to UI constraint, we can't enforce at this level the creation of an element under a choice. We should implements hook that ensure at least one element is selectionned by the user.

done in0.3.0
load left0.000
closed by<not specified>
patch[hooks] Ensure choices are never empty [applied][code gen] Generate a mapping SEDAAlt... -> child_rtype [applied]