Don't issue "No URL" error if BaseURL is set but URL is not

[bpbecker]

HttpCommand will issue a "No URL specified" error if the URL setting is empty. When we implemented the BaseURL setting, in order to make it more convenient to make repeated calls to different endpoints of the same host, the final URL for request is comprised of the concatenation of BaseURL and URL. However, that concatenation isn't performed until after the check for an empty URL is performed. The concatenation should be done prior to the check for an empty URL.