port onchanges to API 8.0 to fix #22

This check seems useless as in framework_agreement get_cheapest_agreement_for_qty only returns an agreement if it found a matching one with enough qty.

@nbessi @lepistone  have we somewhere in OCA modules an override of get_cheapest_agreement_for_qty that could return something else than:

(None, None) or (agreement_id, True) ?