Account for response URL being absent.

According to the documentation[1], this occurs when the response URL is unchanged from the request URL.

[1]: https://cloud.google.com/appengine/docs/java/javadoc/com/google/appengine/api/urlfetch/HTTPResponse#getFinalUrl()
Merge pull request #617 from square/jw/fetch-client-null-url

Account for response URL being absent.