Missing field handling in nested data

[lazargugleta]

Hey,

The schema with a nested struct inside a list causes an error. Related to #257 (assuming - thanks for the quick fix!).
Here is the minimal example:

    schema = pymongoarrow.api.Schema({
        '_id': bson.ObjectId,
        'test_list_struct': [{
            'field1': {
                'sub_field1': pyarrow.string(),
                'sub_field2': pyarrow.string(),
            }
        }],
    })

    temp_collection.insert_one({
        "_id": bson.objectid.ObjectId("000000000000000000000001"),
        "test_list_struct": [
            {"field1": {
                "sub_field1": "test_data",
                # "sub_field2": "test_data",  # missing and causes the error, when uncommented it works fine
            }},
            {"field1": "test_data", }
        ],
    })

    df = pymongoarrow.api.aggregate_polars_all(temp_collection._collection,
                                               schema=schema,
                                               pipeline=[],
                                               )

Produces:

_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
/home/gugleta/.cache/pypoetry/virtualenvs/cxbi-LF4XS2gW-py3.12/lib/python3.12/site-packages/pymongoarrow/api.py:368: in aggregate_polars_all
    return _arrow_to_polars(aggregate_arrow_all(collection, pipeline, schema=schema, **kwargs))
/home/gugleta/.cache/pypoetry/virtualenvs/cxbi-LF4XS2gW-py3.12/lib/python3.12/site-packages/pymongoarrow/api.py:152: in aggregate_arrow_all
    return context.finish()
/home/gugleta/.cache/pypoetry/virtualenvs/cxbi-LF4XS2gW-py3.12/lib/python3.12/site-packages/pymongoarrow/context.py:50: in finish
    array_map = _parse_builder_map(self.manager.finish())
/home/gugleta/.cache/pypoetry/virtualenvs/cxbi-LF4XS2gW-py3.12/lib/python3.12/site-packages/pymongoarrow/context.py:73: in _parse_builder_map
    builder_map[key] = StructArray.from_arrays(arrs, names=names)
pyarrow/array.pxi:4108: in pyarrow.lib.StructArray.from_arrays
    ???
pyarrow/error.pxi:155: in pyarrow.lib.pyarrow_internal_check_status
    ???
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

>   ???
E   pyarrow.lib.ArrowInvalid: Mismatching child array lengths

pyarrow/error.pxi:92: ArrowInvalid

Tested with version 1.6.3
Thanks in advance!

[blink1073]

Thanks for the detailed report! I have a fix in #266.