Allow untagged unit variants to deserialize from Visitor::visit_none()

serde/src/private/de.rs

@@ -2510,6 +2510,13 @@ mod content {
         {
             Ok(())
         }
+
+        fn visit_none<E>(self) -> Result<(), E>
+        where
+            E: de::Error,
+        {
+            Ok(())
+        }
     }
 }