chore: fix return type in ALB multi-query

Signed-off-by: heitorlessa <lessa@amazon.co.uk>

Author: heitorlessa

aws_lambda_powertools/utilities/data_classes/alb_event.py

@@ -32,8 +32,8 @@ def request_context(self) -> ALBEventRequestContext:
         return ALBEventRequestContext(self._data)
 
     @property
-    def multi_value_query_string_parameters(self) -> Optional[Dict[str, List[str]]]:
-        return self.get("multiValueQueryStringParameters")
+    def multi_value_query_string_parameters(self) -> Dict[str, List[str]]:
+        return self.get("multiValueQueryStringParameters") or {}
 
     @property
     def resolved_query_string_parameters(self) -> Dict[str, List[str]]: