Incorrect JSON_KEYS result on string constant

[winoros]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

SELECT JSON_KEYS('[{"A": 1, "B": 2, "C": {"D": 3}},{"A": 1, "B": 2, "C": {"D": 3}}]', '$[last].C');

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

The result should be ["D"].

3. What did you see instead (Required)

NULL returned.

4. What is your TiDB version? (Required)

current master.

[winoros]

It will be a regression when using user variables after #54462 is merged.