fix(admin): allow clearing group policy (#2527)

Co-authored-by: houseme <housemecn@gmail.com>
Co-authored-by: GatewayJ <835269233@qq.com>

rustfs/src/admin/handlers/policies.rs

@@ -377,6 +377,7 @@ impl Operation for RemoveCannedPolicy {
 
 #[derive(Debug, Deserialize, Default)]
 pub struct SetPolicyForUserOrGroupQuery {
+    #[serde(default)]
     #[serde(rename = "policyName", alias = "policy")]
     pub policy_name: String,
     #[serde(rename = "userOrGroup", alias = "user-or-group")]
@@ -983,6 +984,16 @@ mod tests {
         assert!(!query.is_group);
     }
 
+    #[test]
+    fn set_policy_query_allows_missing_policy_name_for_policy_removal() {
+        let query: SetPolicyForUserOrGroupQuery =
+            serde_urlencoded::from_str("userOrGroup=test-group&isGroup=true").expect("query should parse");
+
+        assert!(query.policy_name.is_empty());
+        assert_eq!(query.user_or_group, "test-group");
+        assert!(query.is_group);
+    }
+
     #[test]
     fn policy_association_req_requires_exactly_one_target() {
         let err = validate_policy_association_req(&PolicyAssociationReq {