[BUG] legacy CSV endpoint throws error for `SHOW tables LIKE %`

[joshuali925]

What is the bug?

❯ curl -X POST -k -H 'Content-Type: application/json' localhost:9200/_plugins/_sql?format=csv -d '{"query":"SHOW tables LIKE %;"}'
Cannot invoke "org.opensearch.sql.legacy.executor.csv.CSVResult.getHeaders()" because "result" is null%

normal queries like select * from .kibana works fine on this endpoint

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What is the expected behavior?
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What is your host/environment?

• OS: [e.g. iOS]
• Version [e.g. 22]
• Plugins

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[Yury-Fridlyand]

As of #1181, V2 accepts only quoted literals for system queries (SHOW and DESCRIBE). Probably you need to update your query @joshuali925.