erp5_base: Make sure we do not acquire start_date in listbox_stop_date

for Person_viewAssignment form
5 jobs for arnau in 0 seconds
Status Job ID Name Coverage
  External
failed ERP5.UnitTest-Master

01:38:25

passed SlapOS.Eggs.UnitTest-Master.Python2

00:14:11

passed SlapOS.Eggs.UnitTest-Master.Python3

00:15:54

failed SlapOS.Eggs.UnitTest-Master.Python2

00:13:28

passed SlapOS.Eggs.UnitTest-Master.Python3

00:14:52