fix: preserve NULL semantics in log and power simplification - #24247

…che#24247)

## Which issue does this PR close?

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- Part of apache#24246.

## Rationale for this change

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`log` and `power` simplifications removed a nullable base expression,
which could incorrectly produce a non-NULL result when the base was
NULL.

For example:

```sql
SELECT log(a, 1.0), power(a, 0.0)
FROM (VALUES (NULL::DOUBLE)) AS t(a);
```

These expressions should both return NULL, but simplification could
replace them with 0.0 and 1.0.

## What changes are included in this PR?

only apply these simplifications when the removed base expression is
non-nullable.

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## Are these changes tested?

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Yes. Added sqllogictests covering results and plans.

## Are there any user-facing changes?

Yes. log and power expressions with nullable bases now correctly
preserve NULLs.

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