Handle null Timestamp values

[jonmmease]

Small change to handle null Timestamp values.

Issue: The arrow unary function can be used to map a unary function across a primitive array, propagating the nulls mask. The docs for unary state that the function must be infallible because it will be evaluated with whatever happens to be stored in the data array for null values. The issue is that timestamp_to_timestamptz UDF performs a timezone conversion, and so it is not valid for arbitrary i32 array elements.

Solution: Switch to using array.iter().map() to map over boxed Optional array element values. This way we only perform the timezone conversions on valid timestamp values.