[FIX] payment_paypal: avoid crash in validation without reference

When `paypal_validate_data` is called without any reference, `tx` is None.
Before the migration, it was catched by a `tx and tx.acquired_id` but it is no
longer the case.

Introduced at f1ac1b12

Fixes #13834

addons/payment_paypal/controllers/main.py

@@ -56,7 +56,7 @@ class PaypalController(http.Controller):
         tx = None
         if reference:
             tx = request.env['payment.transaction'].search([('reference', '=', reference)])
-        paypal_urls = tx.acquirer_id._get_paypal_urls(tx.acquirer_id.environment or 'prod')
+        paypal_urls = request.env['payment.acquirer']._get_paypal_urls(tx and tx.acquirer_id.environment or 'prod')
         pdt_request = bool(new_post.get('amt'))  # check for spefific pdt param
         if pdt_request:
             # this means we are in PDT instead of DPN like before