[FIX] payment_paypal: paypal IPN, receiver_id is not always passed

The method  _paypal_form_get_invalid_parameters checks that the params passed by the paypal feedback coroborate with the transaction requested by Odoo.

It looks like the 'receiver_id' is not always passed by the paypal feedback. Therefore, we check this param only if it is available in the paypal feeback.

This issue had as side-effect to not validate the paypal transaction, which remained 'draft' permanently.