LEAD/LAG IGNORE NULLS panics without a null bitmap

[xudong963]

Describe the bug

The whole-partition evaluation path for LEAD and LAG with IGNORE NULLS calls array.nulls().unwrap(). Arrow arrays containing no null values commonly omit the null bitmap, so evaluating either function on such an array panics.

To Reproduce

Evaluate LEAD or LAG with IGNORE NULLS through PartitionEvaluator::evaluate_all using an array constructed without nulls, for example:

let array = Int32Array::from(vec![1, 2, 3]);
assert!(array.nulls().is_none());

The evaluation panics with:

called `Option::unwrap()` on a `None` value

Expected behavior

IGNORE NULLS should behave like the regular shift when the input contains no null values, returning the expected LEAD or LAG result without requiring a null bitmap.

Additional context

The panic is in evaluate_all_with_ignore_null in datafusion/functions-window/src/lead_lag.rs.