routerrpc: reject payment to invoice that don't have payment secret or blinded paths

Ensure that a payment is only sent if the invoice includes either a
payment address (payment secret) or at least one blinded path.

Author: erickcestari

lnrpc/routerrpc/router_backend.go

@@ -985,6 +985,14 @@ func (r *RouterBackend) extractIntentFromSendRequest(
 			return nil, err
 		}
 
+		// An invoice must include either a payment address or
+		// blinded paths.
+		if payReq.PaymentAddr.IsNone() &&
+			len(payReq.BlindedPaymentPaths) == 0 {
+			return nil, errors.New("payment request must contain " +
+				"either a payment address or blinded paths")
+		}
+
 		// If the amount was not included in the invoice, then we let
 		// the payer specify the amount of satoshis they wish to send.
 		// We override the amount to pay with the amount provided from