[Python] Implement unification of null dictionaries

### Describe the enhancement requested

Currently, if you try to do something like:

```python
a = pa.array([None, None, None]).cast(pa.dictionary(pa.int8(), pa.null()))
b = pa.array([None, None, None]).cast(pa.dictionary(pa.int8(), pa.null()))

pa.chunked_array([a, b]).unify_dictionaries()
```

PyArrow will raise an exception like:
```
File pyarrow/table.pxi:1206, in pyarrow.lib.ChunkedArray.unify_dictionaries()

File pyarrow/error.pxi:154, in pyarrow.lib.pyarrow_internal_check_status()

File pyarrow/error.pxi:91, in pyarrow.lib.check_status()

ArrowNotImplementedError: Unification of null dictionaries is not implemented
```

Is there any way to just implement this? Manually calling `.combine_chunks()` seems to work fine, so the logic is clearly implemented somewhere.

```
a = pa.array([None, None, None]).cast(pa.dictionary(pa.int8(), pa.null()))
b = pa.array([None, None, None]).cast(pa.dictionary(pa.int8(), pa.null()))

pa.chunked_array([a, b]).combine_chunks()
```

I admit this might be a very niche feature (I am confused why I have feather files with these dictionary-encoded nulls, but I do and it'd be nice to be able to handle them).

### Versions

I am testing this on Python 3.10 and pyarrow 15.

### Component(s)

Python