Handle None values properly when CLI output is YAML/JSON format (#13024)

str() should not be applied to None when we 'normalize' the data before we print it,
otherwise it's not given in proper format in YAML/JSON format of the CLI output

airflow/cli/simple_table.py

@@ -60,6 +60,8 @@ def _normalize_data(self, value: Any, output: str) -> Union[list, str, dict]:
             return [self._normalize_data(x, output) for x in value]
         if isinstance(value, dict) and output != "table":
             return {k: self._normalize_data(v, output) for k, v in value.items()}
+        if value is None:
+            return None
         return str(value)
 
     def print_as(self, data: List[Union[Dict, Any]], output: str, mapper: Optional[Callable] = None):