fix negative array subscripts should not index from the end

[xiedeyantu]

Which issue does this PR close?

• Closes PostgreSQL compatibility: negative array subscripts should not index from the end #22254

Rationale for this change

PostgreSQL treats negative array subscripts as out of bounds and returns NULL. DataFusion was incorrectly interpreting negative subscripts as indexing from the end of the array, which caused behavioral mismatch.

What changes are included in this PR?

• Updated array_element / array subscript evaluation to treat non-positive indexes as out of bounds and return NULL.
• Updated sqllogictest coverage for array subscript and array_element behavior, including the [-1] case.

Are these changes tested?

Yes.

Validated with:

• cargo test -p datafusion-functions-nested test_array_element_non_positive_indexes_return_null
• cargo test -p datafusion-sqllogictest --test sqllogictests array_index
• cargo test -p datafusion-sqllogictest --test sqllogictests expr

Are there any user-facing changes?

Yes. Array subscripts with negative indexes now return NULL instead of indexing from the end, aligning DataFusion behavior with PostgreSQL.