src/rss_converter_twitter.com.xsl: only output channel image when it's available

Hashtag pages do not have an image usable as a channel logo, and in
cases like this the <url/> element would be empty, but this would make
the feed invalid according to https://www.feedvalidator.org

So, to produce feeds which validate, avoid outputting the whole <image/>
element when there is no suitable image to use as a channel logo.