rpad/lpad panic (Option::unwrap on None) when the length column contains NULL

[peterxcli]

Describe the bug

spark_read_side_padding_internal (native/spark-expr/src/static_invoke/char_varchar_utils/read_side_padding.rs:225) iterates the length array with let length = length.unwrap();, so any NULL in the length column panics with called Option::unwrap() on a None value and fails the task with a CometNativeException. Spark's StringRPad / StringLPad are NullIntolerant and return NULL for that row.

The path is default-on: CometStringRPad / CometStringLPad only require the pad to be a literal, so rpad(s, len_col) runs natively. Both the 2-arg [Array, Array] arm (lines 144-162) and the 3-arg [Array, Array, Scalar] arm (163-183) reach the unwrap.

Steps to reproduce

CREATE TABLE t USING parquet AS SELECT * FROM VALUES ('abc', 5), ('abc', CAST(NULL AS INT)) AS t(s, l);
SELECT rpad(s, l) FROM t;      -- same for lpad(s, l) and rpad(s, l, 'x')

Spark: abc , NULL. Comet: task fails — called Option::unwrap() on a None value at read_side_padding.rs:225.

Unit level: spark_rpad(["abc","abc"], lengths=[Some(4), None], " ") panics.

Expected behavior

NULL for every row whose length is NULL, like Spark.

Proposed solution

Replace the unwrap with a match that appends null (and continues) when either the string or the length is null, in both arms. Add a unit test with a NULL length and a NULL-length row to string_rpad.sql / testStringPadding.

Additional context

The fuzz test cannot catch this because FuzzDataGenerator never generates NULL integers (#5389). The unwrap was introduced in #2630; #2096 was a different, earlier rpad panic (non-literal length hitting the unsupported-arguments arm).