PaymentRequest: Remove unnecessary code

The hasError() method is already calling IsNull() internally.
Also, the error field in mojom has a default value as empty string.

Bug: none
Change-Id: I98edd7c11a32379cc477b3f0520b3293b83a9009
Reviewed-on: https://chromium-review.googlesource.com/1239294Reviewed-by: Rouslan Solomakhin <rouslan@chromium.org>
Commit-Queue: Jinho Bang <jinho.bang@samsung.com>
Cr-Commit-Position: refs/heads/master@{#593560}

third_party/blink/renderer/modules/payments/payment_request.cc

@@ -675,7 +675,7 @@ void ValidateAndConvertPaymentDetailsUpdate(const PaymentDetailsUpdate& input,
       return;
   }
 
-  if (input.hasError() && !input.error().IsNull()) {
+  if (input.hasError()) {
     String error_message;
     if (!PaymentsValidators::IsValidErrorMsgFormat(input.error(),
                                                    &error_message)) {
@@ -683,8 +683,6 @@ void ValidateAndConvertPaymentDetailsUpdate(const PaymentDetailsUpdate& input,
       return;
     }
     output->error = input.error();
-  } else {
-    output->error = "";
   }
 
   if (input.hasShippingAddressErrors()) {