Improve party_required test

Following the #4292 (closed), the party_required tests should not badly fail if the value is None.

assigned to @ced

added accounting type::behavior + 1 deleted label

New changeset a04ca9194085 by C?dric Krier in branch 'default':
Make party_required test works with None value
http://hg.tryton.org/modules/account/rev/a04ca9194085

added 1 deleted label and removed 1 deleted label

closed

New changeset 900e7cf9c990 by C?dric Krier in branch '3.4':
Make party_required test works with None value
http://hg.tryton.org/modules/account/rev/900e7cf9c990