check for description length before subscripting (#90)

sphinxext/opengraph/descriptionparser.py

@@ -101,7 +101,7 @@ def dispatch_departure(self, node: nodes.Element) -> None:
 
         # Separate end of list from text
         if isinstance(node, nodes.Sequential):
-            if self.description[-1] == ",":
+            if self.description and self.description[-1] == ",":
                 self.description = self.description[:-1]
             self.description += "."
             self.list_level -= 1