Concatenating dictionaries that have nulls in them yields strange results

[westonpace]

Describe the bug
If I concatenate two dictionaries that have nulls in the dictionary (not the indices) then I sometimes get garbage back.

To Reproduce

    #[test]
    pub fn test_dict_concat_two() {
        let keys1 = UInt16Array::from_iter_values(vec![2, 2]);
        let keys2 = UInt16Array::from_iter_values(vec![1]);
        let dict1 = StringArray::from(vec![Some("a"), Some("b"), None]);
        let dict2 = StringArray::from(vec![Some("c"), Some("d"), None]);

        let arr1 = DictionaryArray::new(keys1, Arc::new(dict1));
        let arr2 = DictionaryArray::new(keys2, Arc::new(dict2));

        println!("{:?}", arr1);
        println!("{:?}", arr2);
        let combined = arrow_select::concat::concat(&[&arr1, &arr2]).unwrap();
        println!("{:?}", combined);

        let expected = StringArray::from(vec![None, None, Some("d")]);
        let actual = arrow_cast::cast(combined.as_ref(), &DataType::LargeUtf8).unwrap();

        assert_eq!(actual.as_ref(), &expected);
    }

Expected behavior
The test in the reproduction passes.

Additional context
The output from the test is:

DictionaryArray {keys: PrimitiveArray<UInt16>
[
  2,
  2,
] values: StringArray
[
  "a",
  "b",
  null,
]}

DictionaryArray {keys: PrimitiveArray<UInt16>
[
  1,
] values: StringArray
[
  "c",
  "d",
  null,
]}

DictionaryArray {keys: PrimitiveArray<UInt16>
[
  0,
  0,
  0,
] values: StringArray
[
  "d",
]}

thread 'data::tests::test_dict_concat_two' panicked at rust/lance-encoding/src/data.rs:1179:9:
assertion `left == right` failed
  left: LargeStringArray
[
  "d",
  "d",
  "d",
]
 right: StringArray
[
  null,
  null,
  "d",
]