You are parsing as HTML, but the source is XML. In HTML the link
element is empty so Nokogiri parses is as something like <link></link>http://example.com ...
where the url is a text node outside the link
element. When you then query the parsed document the link
elements are empty.
To fix it you should parse as XML:
doc = Nokogiri::XML(open('http://www.kffl.com/printRSS.php/NFL-ARI'))
# ...