When the artist name couldn't found for a Newgrounds URL, for example
for `https://www.newgrounds.com/dump/item`, then the `profile_url`
method erroneously returned `https://.newgrounds.com`. This led to an
error later on when the artist finder tried to parse the invalid URL.
Also fix `strategy_should_work` to test that the profile URL is a valid
URL, and not to try to download the file when image_urls is empty.
