fix(agg): fix first_value and last_value to not ignore NULLs

[stdrc]

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What's changed and what's your intention?

To align behavior with PG, we should not ignore NULLs when computing first_value and last_value.

This PR affects batch aggregate, batch over window. Also, a previous PR #19233 has already changed the behavior of streaming over window to the correct one.

Fixes #18927.

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[stdrc]

• fix(agg): fix first_value and last_value to not ignore NULLs #19275 👈
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