fix: update user empty ukey_sn field value

apps/authentication/backends/ukey/vendors/ji_da/sdk_config.yaml

@@ -297,7 +297,7 @@ operations:
           url_format: 
             user_id: "{{ user.id }}"
           body:
-            ukey_sn: null
+            ukey_sn: ""
           register: user
         
     - key: issueCert

apps/authentication/backends/ukey/vendors/long_mai/sdk_config.yaml

@@ -359,7 +359,7 @@ operations:
           url_format: 
             user_id: "{{ user.id }}"
           body:
-            ukey_sn: null
+            ukey_sn: ""
           register: user
         
     - key: issueCert

apps/users/serializers/user.py

@@ -325,6 +325,14 @@ class UserSerializer(
                 attrs.pop(field, None)
         return attrs
 
+    @staticmethod
+    def clean_ukey_fields(attrs):
+        for field in ("ukey_sn",):
+            value = attrs.get(field)
+            if value is None:
+                attrs.pop(field, None)
+        return attrs
+
     def check_disallow_self_update_fields(self, attrs):
         request = self.context.get("request")
         if not request or not request.user.is_authenticated:
@@ -348,6 +356,7 @@ class UserSerializer(
         attrs = self.check_disallow_self_update_fields(attrs)
         attrs = self.change_password_to_raw(attrs)
         attrs = self.clean_auth_fields(attrs)
+        attrs = self.clean_ukey_fields(attrs)
         password_strategy = attrs.pop("password_strategy", None)
         request = get_current_request()
         if request: