Improve the command JSON.MERGE via jsonpath::remove

[2rueSid]

Updated JSON.MERGE command

1. Handle null case with jsoncons::jsonpath::remove(value, path);`
2. Handle non-existing key Create an object from path danielaparker/jsoncons#468

@PragmaTwice can you please review PR.

Also, I can update SET command to handle non-existing key, as in this example:

redis> JSON.SET doc $ '{"a":2}'
OK
redis> JSON.SET doc $.b '8'
OK
redis> JSON.GET doc $
"[{\"a\":2,\"b\":8}]"

[sonarcloud]

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[PragmaTwice]

LGTM