ox-html.el: Fix bug introduced in 077a24b

* ox-html.el (org-html-format-list-item): Check that contents in
  non-nil.

lisp/ox-html.el

@@ -2524,7 +2524,7 @@ INFO is a plist holding contextual information.  See
 			  class (concat checkbox term))
 		  "<dd>"))))
      (unless (eq type 'descriptive) checkbox)
-     (org-trim contents)
+     (and contents (org-trim contents))
      (case type
        (ordered "</li>")
        (unordered "</li>")