Fix private parameters wrapping when deserializing JSON

On the Python side it was assumed that the blacklisted private
parameters were always dictionaries, but since they're optional, they
could be 'None' as well.
Signed-off-by: Petr Pudlak <pudlak@google.com>
Reviewed-by: Klaus Aehlig <aehlig@google.com>

lib/serializer.py

@@ -111,7 +111,7 @@ def WrapPrivateValues(json):
         if field in constants.PRIVATE_PARAMETERS_BLACKLIST:
           if not field.endswith("_cluster"):
             data[field] = PrivateDict(value)
-          else:
+          elif data[field] is not None:
             for os in data[field]:
               value[os] = PrivateDict(value[os])
         else: