Url attribute is not used as content for empty ulink elements

When using an ulink element as follows in a docbook document:

<ulink url="http://some/url"/>

The original docbook stylesheets use "http://some/url" as text for the link (something like <a href="http://some/url">http://some/url&lt;/a&gt;), but the simplified forrest stylesheets create a link with no text (something like <a href="http://some/url"></a>.

This patch fixes that.