test(serde): add unit tests for serde optional (#1658)

Co-authored-by: zerosnacks <[email protected]>

crates/serde/src/optional.rs

@@ -26,3 +26,53 @@ where
 
     Ok(value)
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use serde_json::json;
+
+    #[derive(Debug, Deserialize, PartialEq)]
+    struct TestStruct {
+        #[serde(default, deserialize_with = "null_as_default")]
+        value: Vec<i32>,
+
+        #[serde(default, deserialize_with = "reject_if_some")]
+        should_be_none: Option<String>,
+    }
+
+    #[test]
+    fn test_null_as_default_with_null() {
+        let json_data = json!({ "value": null });
+        let result: TestStruct = serde_json::from_value(json_data).unwrap();
+        assert_eq!(result.value, Vec::<i32>::new());
+    }
+
+    #[test]
+    fn test_null_as_default_with_value() {
+        let json_data = json!({ "value": [1, 2, 3] });
+        let result: TestStruct = serde_json::from_value(json_data).unwrap();
+        assert_eq!(result.value, vec![1, 2, 3]);
+    }
+
+    #[test]
+    fn test_null_as_default_with_missing_field() {
+        let json_data = json!({});
+        let result: TestStruct = serde_json::from_value(json_data).unwrap();
+        assert_eq!(result.value, Vec::<i32>::new());
+    }
+
+    #[test]
+    fn test_reject_if_some_with_none() {
+        let json_data = json!({});
+        let result: TestStruct = serde_json::from_value(json_data).unwrap();
+        assert_eq!(result.should_be_none, None);
+    }
+
+    #[test]
+    fn test_reject_if_some_with_some() {
+        let json_data = json!({ "should_be_none": "unexpected value" });
+        let result: Result<TestStruct, _> = serde_json::from_value(json_data);
+        assert!(result.is_err());
+    }
+}