Prevent rendering of excessive <li> wrapper around a category breadcrumb which already is a <li>.

The excessive <li> wrapper coming from the template is automatically closed by the browser during the HTML parsing step, causing an empty <li> being visible in the rendered source (preceding the first breadcrumb). This causes a visual bug when not using the default Discourse stylesheet, because the extra LI element pushes the content to the right and sometimes also renders an extra black pixel.
Merge pull request #2160 from borhub/fix-excessive-li-in-category-breadcrumb

Fix visual bug from excessive LIs being rendered around Category breadcrumbs