cards changed from None to default_factory

entities/json/pa_message.py

 """ PA message to send to Teams """
-from dataclasses import dataclass
+from dataclasses import dataclass, field
 from typing import Optional, Union, List, Dict
 
 from entities.json.camel_case_mixin import CamelCaseMixin
@@ -10,6 +10,6 @@ class PAMessage(CamelCaseMixin):
     """ PA message Schema """
     conversation_id: str
     tenant_id: str
-    text: Optional[str]
+    text: Optional[str] = None
     card: Optional[Dict] = None
-    cards: Optional[List[Dict]] = None
+    cards: Optional[List[Dict]] = field(default_factory=list)