fix: preserve NULL semantics in log and power simplification

[lyne7-sc]

Which issue does this PR close?

• Part of Expression simplifications may produce incorrect results with NULLs #24246.

Rationale for this change

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

For example:

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.

Are these changes tested?

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.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 69.23077% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.01%. Comparing base (4b48cb0) to head (3a74506).
⚠️ Report is 32 commits behind head on main.

Files with missing lines	Patch %	Lines
datafusion/functions/src/math/log.rs	71.42%	0 Missing and 2 partials ⚠️
datafusion/functions/src/math/power.rs	66.66%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #24247      +/-   ##
==========================================
- Coverage   81.06%   81.01%   -0.05%     
==========================================
  Files        1106     1106              
  Lines      382252   383982    +1730     
  Branches   382252   383982    +1730     
==========================================
+ Hits       309861   311080    +1219     
- Misses      54083    54561     +478     
- Partials    18308    18341      +33     

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[alamb]

Thanks @lyne7-sc