Fix issues in multi-key json case and empty value case

[PHILO-HE]

For multi-key case, get_json_object('$.field_name') should still get correct value. E.g., {"a": 1, "b", 2}, get_json_object('$.b') should return 2. Without this patch, 1 will be incorrectly returned.
For empty value case, e.g., {"a": "", "b", 2}, get_json_object('$.a') should output "" instead of null.

[github-actions]

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ARROW-${JIRA_ID}: [${COMPONENT}] ${SUMMARY}

or

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See also:

• Other pull requests
• Contribution Guidelines - How to contribute patches

[PHILO-HE]

Test result: