json_path_exists can't deal with null json

[Kev1n8]

What type of bug is this?

Incorrect result

What subsystems are affected?

Standalone mode, Frontend

Minimal reproduce step

The recently added json_path_exists behaves wrongly when json param is null.
For example:

SELECT json_path_exists(parse_json('null'), '$.a')
SELECT json_paht_exists(parse_json('{}'),'null')

OUTPUT:
false
false

What did you expect to see?

According to JSON_PATH_MATCH, the result should be null.

What did you see instead?

Error.

What operating system did you use?

macOS 15.0.1

What version of GreptimeDB did you use?

0.9.5

Relevant log output and stack trace

No response

[Kev1n8]

I'm already working on it.

[Kev1n8]

As discussed at here, we will keep the param non-nullable but return null when json is parse_json('null').