Describe the bug

DictionaryArray::is_null(i) currently returns false for entries where the dictionary index is valid, even if the value it points to in the dictionary value array is null. This results in incorrect behavior when dictionary-encoded arrays are used in higher-level operations like count(distinct ...), which rely on is_null to exclude nulls.

This violates expected null semantics — the dictionary key is not null, but the resolved value is — which should still be treated as null.

To Reproduce

use arrow::array::{ArrayRef, DictionaryArray, Int32Array, StringArray};
use std::sync::Arc;

fn main() {
    let dict_values = StringArray::from(vec![None, Some("abc")]);
    let dict_indices = Int32Array::from(vec![0, 0, 0, 0, 0]); // All indices point to a null value
    let dict = DictionaryArray::new(dict_indices, Arc::new(dict_values) as ArrayRef);

    for i in 0..dict.len() {
        println!("is_null({}) = {}", i, dict.is_null(i));
    }
}

Additional context

First raised here - apache/datafusion#16228