FUEL: remove empty (nil) lists of parsed words

FUEL interprets nil as f, therefore they need to be filtered out to
prevent empty lists of words from appearing as an "f" character.

misc/fuel/fuel-markup.el

@@ -423,7 +423,7 @@
       (let ((elems '(($heading "Words"))))
         (push (fuel-markup--parse-classes) elems)
         (push (fuel-markup--parse-words) elems)
-        (reverse elems)))))
+        (reverse (remove nil elems))))))
 
 (defun fuel-markup--describe-words (e)
   (when (cadr e)