fix: Refines None checks in result transformation

Simplifies the code by replacing type checks for None with
direct comparisons, improving readability and consistency in
handling None values during output validation.

Signed-off-by: -LAN- <laipz8200@outlook.com>

api/core/workflow/nodes/code/code_node.py

             if output_config.type == "object":
                 # check if output is object
                 if not isinstance(result.get(output_name), dict):
-                    if isinstance(result.get(output_name), type(None)):
+                    if result.get(output_name) is None:
                         transformed_result[output_name] = None
                     else:
                         raise OutputValidationError(
             elif output_config.type == "array[number]":
                 # check if array of number available
                 if not isinstance(result[output_name], list):
-                    if isinstance(result[output_name], type(None)):
+                    if result[output_name] is None:
                         transformed_result[output_name] = None
                     else:
                         raise OutputValidationError(
             elif output_config.type == "array[string]":
                 # check if array of string available
                 if not isinstance(result[output_name], list):
-                    if isinstance(result[output_name], type(None)):
+                    if result[output_name] is None:
                         transformed_result[output_name] = None
                     else:
                         raise OutputValidationError(
             elif output_config.type == "array[object]":
                 # check if array of object available
                 if not isinstance(result[output_name], list):
-                    if isinstance(result[output_name], type(None)):
+                    if result[output_name] is None:
                         transformed_result[output_name] = None
                     else:
                         raise OutputValidationError(