The `JSON_SET` returns wrong result for empty object

[YangKeao]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

SELECT JSON_SET('{}', '$[last]', 'end')

2. What did you expect to see? (Required)

["\"end\""]

3. What did you see instead (Required)

["[{}, \"end\"]"]

4. What is your TiDB version? (Required)