[Payments] Don't print empty URL in warning.

Before this patch, missing HTTP Link header for payment method manifest
would result in the following error messages in console:

  Unable to make a HEAD request to https://foo for payment method manifest.
   is not a valid payment method manifest URL.

This patch hides the second error message when the payment method
manifest URL is empty.

After this patch, missing HTTP Link header for payment method manifest
results in the following error message in console:

  Unable to make a HEAD request to https://foo for payment method manifest.

Bug: NONE
Change-Id: I83adced7779b692e81395c660e706f18269c1357
Reviewed-on: https://chromium-review.googlesource.com/962879Reviewed-by: anthonyvd <anthonyvd@chromium.org>
Commit-Queue: Rouslan Solomakhin <rouslan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543141}

components/payments/core/payment_manifest_downloader.cc

@@ -128,7 +128,10 @@ void PaymentManifestDownloader::OnURLFetchComplete(
       InitiateDownload(url, net::URLFetcher::GET,
                        std::move(download->callback));
     } else {
-      LOG(ERROR) << url << " is not a valid payment method manifest URL.";
+      // If the URL is empty, then ParseResponseHeader() has already printed an
+      // explanation.
+      if (!url.is_empty())
+        LOG(ERROR) << url << " is not a valid payment method manifest URL.";
       std::move(download->callback).Run(std::string());
     }
   } else {