Deserialize only if payload is not None (#48)

Deserialize only if payload is not None. Fixes #47

Author: nikhilym

ask-sdk-core/ask_sdk_core/serialize.py

@@ -143,6 +143,9 @@ def __deserialize(self, payload, obj_type):
         :return: deserialized object
         :rtype: T
         """
+        if payload is None:
+            return None
+
         if type(obj_type) == str:
             if obj_type.startswith('list['):
                 # Get object type for each item in the list