Account for nodes without children in texinfo's ignores

This prevented compilation of the pytorch tutorial at
https://github.com/pytorch/tutorials, with the latest master.

Author: quotuva

sphinx/writers/texinfo.py

@@ -691,7 +691,7 @@ def visit_reference(self, node: Element) -> None:
         # cases for the sake of appearance
         if isinstance(node.parent, (nodes.title, addnodes.desc_type)):
             return
-        if isinstance(node[0], nodes.image):
+        if len(node) != 0 and isinstance(node[0], nodes.image):
             return
         name = node.get('name', node.astext()).strip()
         uri = node.get('refuri', '')