Doc: Fix detection whether documentation is empty

library/Icinga/Data/Tree/TreeNodeIterator.php

@@ -84,4 +84,14 @@ class TreeNodeIterator implements RecursiveIterator
     {
         return new static($this->current());
     }
+
+    /**
+     * Get whether the iterator is empty
+     *
+     * @return bool
+     */
+    public function isEmpty()
+    {
+        return empty($this->children);
+    }
 }

modules/doc/library/Doc/Renderer/DocTocRenderer.php

@@ -70,7 +70,7 @@ class DocTocRenderer extends DocRenderer
      */
     public function render()
     {
-        if (empty($this->content)) {
+        if ($this->getInnerIterator()->isEmpty()) {
             return '<p>' . mt('doc', 'Documentation is empty.') . '</p>';
         }
         $view = $this->getView();