fix: handle empty message case in encryption function

--bug=1053881 --user=刘瑞斌 【函数库】-编辑函数，修改启动参数的必填选项后，保存函数报错 https://www.tapd.cn/57709429/s/1675682

apps/function_lib/serializers/function_lib_serializer.py

@@ -44,6 +44,8 @@ def encryption(message: str):
     :param message:
     :return:
     """
+    if message == "":
+        return ""
     max_pre_len = 8
     max_post_len = 4
     message_len = len(message)