slapos_payzen&wechat: Change payment_mode if invoice/payment price is zero

   It isn't possible to pay on payzen zero amount invoices, so just update payment_mode to skip the payment from the search (to prevent bloat activities by relaunching activities that will never change the state of the object).
1 job for master in 0 seconds
Status Job ID Name Coverage
  External
failed SlapOS-Master.UnitTest-Rafael

01:27:08