Fix: SparkAscii nullability to depend on input nullability

[Yuvraj-cyborg]

Closes #19169

Rationale for this change:

The current implementation of SparkAscii UDF uses the default is_nullable which always returns true. This is incorrect because the output should only be nullable if the input argument is nullable. This change implements proper null propagation behavior by using return_field_from_args .

Changes in PR:

• Implemented return_field_from_args for SparkAscii to properly compute output nullability based on input argument nullability
• Changed return_type to return internal_err! since return_field_from_args is now used (following the pattern used by other Spark functions like ilike, concat, elt)
• Added unit tests verifying the nullability behavior:
  • Output is nullable when input is nullable
  • Output is non-nullable when input is non-nullable

Test Coverage:

Yes, tests are included to verify the change.

User-facing Changes:

No user-facing changes.

[Yuvraj-cyborg]

cc: @Jefffrey @rluvaton

[Jefffrey]

Thanks @Yuvraj-cyborg