`Operator::returns_null_on_null()` incorrectly classifies string concatenation

[neilconway]

Describe the bug

Operator::returns_null_on_null() claims string concatenation produces an empty string for NULL inputs; this is not the case. (The concat UDF has the weird NULL -> empty string behavior; the concatenation operator (||) does not.)

To Reproduce

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Expected behavior

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Additional context

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[neilconway]

take