[IMPALA-7750] Prune trivial ELSE clause in CASE simplification - ASF JIRA

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A trivial optimization is to omit ELSE if it adds no value:

CASE WHEN id = 10 THEN id ELSE NULL END

The ELSE case defaults to null if not provided, so the above can be rewritten to:

CASE WHEN id = 10 THEN id END

Also, the simplification can omit the trailing WHEN clause if it returns NULL. For example:

CASE WHEN id = 0 THEN 10 WHEN id = 1 THEN NULL END

Should be simplified to:

CASE WHEN id = 0 THEN 10 END

is part of

IMPALA-7747 Clean up the Expression Rewriter