fix(spark): return NULL from char for a NULL scalar argument

[ranflarion]

Which issue does this PR close?

• Closes Spark char fails on a NULL scalar argument #26125.

Rationale for this change

SELECT char(NULL::INT) fails with Execution error: The argument must be an Int64 array or scalar. where Spark returns NULL. NULL rows of an array argument already return NULL, only a NULL scalar argument reaches the error.

What changes are included in this PR?

spark_chr returns a NULL Utf8 scalar for a NULL Int64 scalar.

What is the testing strategy for this PR?

A char(NULL::INT) case in spark/string/char.slt, which fails on main with the error above.

Are there any user-facing changes?

char of a NULL scalar returns NULL instead of an error.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.73%. Comparing base (1c43063) to head (55bf1e3).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff            @@
##             main   #26126    +/-   ##
========================================
  Coverage   82.73%   82.73%            
========================================
  Files        1147     1147            
  Lines      449389   449541   +152     
  Branches   449389   449541   +152     
========================================
+ Hits       371788   371923   +135     
- Misses      54943    54944     +1     
- Partials    22658    22674    +16     

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[neilconway]

Thanks @ranflarion! lgtm.

I notice that we don't declare is_strict for char, or indeed any of the other Spark UDFs. That would be good to do at some point but is orthogonal.