Empty `contents` are encoded to `null`

[kyu-suke]

System Informations

• Go version: go1.22.5 darwin/arm64
• OS: macOS 14.4.1

Expected Behavior

{
    "type": "flex",
    "contents": {
        "type": "bubble",
        "header": {
            "type": "box",
            "layout": "vertical",
            "contents": []
        },
        "body": {
            "type": "box",
            "layout": "vertical",
            "contents": [
                {
                    "type": "box",
                    "layout": "baseline",
                    "contents": [],
                    "spacing": "sm"
                },
                {
                    "type": "box",
                    "layout": "vertical",
                    "contents": [
                        {
                            "type": "box",
                            "layout": "baseline",
                            "contents": [],
                            "spacing": "sm"
                        }
                    ]
                }
            ]
        }
    }
}

↓

{
    "type": "flex",
    "altText": "",
    "contents": {
        "type": "bubble",
        "header": {
            "layout": "vertical",
            "flex": 0,
            "type": "box",
            "contents": []
        },
        "body": {
            "layout": "vertical",
            "flex": 0,
            "type": "box",
            "contents": [
                {
                    "type": "box",
                    "layout": "baseline",
                    "flex": 0,
                    "contents": null,
                    "spacing": "sm"
                },
                {
                    "type": "box",
                    "layout": "vertical",
                    "flex": 0,
                    "contents": [
                        {
                            "type": "box",
                            "layout": "baseline",
                            "flex": 0,
                            "contents": [], // is should empty?
                            "spacing": "sm"
                        }
                    ]
                }
            ]
        }
    }
}

Current Behavior

The contents value of type box of flex message contents is encoded to null when it is nested and empty. So narrowcast API returns May not be empty.
I think it is necessary to modify the unmarshal logic in linebot/messaging_api/model_flex_box.go.
If this approach is acceptable, I would like to proceed with submitting a pull request. Please let me know if there are any concerns or suggestions for improvement.

{
    "type": "flex",
    "altText": "",
    "contents": {
        "type": "bubble",
        "header": {
            "layout": "vertical",
            "flex": 0,
            "type": "box",
            "contents": []
        },
        "body": {
            "layout": "vertical",
            "flex": 0,
            "type": "box",
            "contents": [
                {
                    "type": "box",
                    "layout": "baseline",
                    "flex": 0,
                    "contents": null,
                    "spacing": "sm"
                },
                {
                    "type": "box",
                    "layout": "vertical",
                    "flex": 0,
                    "contents": [
                        {
                            "type": "box",
                            "layout": "baseline",
                            "flex": 0,
                            "contents": null, // `[]` is encoded to `null`
                            "spacing": "sm"
                        }
                    ]
                }
            ]
        }
    }
}

Steps to Reproduce

flex2 := `{
    "type": "flex",
    "contents": {
        "type": "bubble",
        "header": {
            "type": "box",
            "layout": "vertical",
            "contents": []
        },
        "body": {
            "type": "box",
            "layout": "vertical",
            "contents": [
                {
                    "type": "box",
                    "layout": "baseline",
                    "contents": [],
                    "spacing": "sm"
                },
                {
                    "type": "box",
                    "layout": "vertical",
                    "contents": [
                        {
                            "type": "box",
                            "layout": "baseline",
                            "contents": [],
                            "spacing": "sm"
                        }
                    ]
                }
            ]
        }
    }
}`

f2, err := messaging_api.UnmarshalMessage([]byte(flex2))
if err != nil {
	fmt.Println(err)
}
ff2, _ := json.MarshalIndent(f2, "", "    ")
fmt.Println(string(ff2))

[habara-k]

Thank you for using line-bot-sdk-go.

We appreciate your report. We have also confirmed the behavior on our end. It seems that an empty array is being unmarshaled as null.

We believe this behavior should be corrected. It would be greatly appreciated if you could create a PR to fix this issue. Thank you in advance for your cooperation.