Function case_conversion_ascii_array 

fn case_conversion_ascii_array<'a, O, F>(
    string_array: &'a GenericStringArray<O>,
    op: F,
) -> Result<ArrayRef>
where
    O: OffsetSizeTrait,
    F: Fn(&'a str) -> String,

All values of string_array are ASCII, and when converting case, there is no changes in the byte array length. Therefore, the StringArray can be treated as a complete ASCII string for case conversion, and we can reuse the offsets buffer and the nulls buffer.