minor(dev): dstc3 nan fix

daiso.py

@@ -923,6 +923,10 @@ LABELS_MAPPING = {
         "deny": {
             "base": "Deny",
             "ISO": "answer"
+        },
+        None: {
+            "base": "None",
+            "ISO": None
         }
     },
     "dstc8-sgd": {