Render empty <ul><li><ul> correctly

* lisp/net/shr.el (shr-tag-ul): Render empty <ul><li><ul>
correctly (bug#22964).
(cherry picked from commit 4f6ea3988b)

Backport:

lisp/net/shr.el

@@ -1480,6 +1480,10 @@ The preference is a float determined from `shr-prefer-media-type'."
   (shr-ensure-paragraph)
   (let ((shr-list-mode 'ul))
     (shr-generic dom))
+  ;; If we end on an empty <li>, then make sure we really end on a new
+  ;; paragraph.
+  (unless (bolp)
+    (insert "\n"))
   (shr-ensure-paragraph))
 
 (defun shr-tag-ol (dom)