test_additional_extension_arrays fails with pd.NA
cc @jorisvandenbossche 

im using pyarrow 0.15.1 locally

```
_____________________________________________ TestParquetPyArrow.test_additional_extension_arrays _____________________________________________

self = <pandas.tests.io.test_parquet.TestParquetPyArrow object at 0x129bc4050>, pa = 'pyarrow'

    @td.skip_if_no("pyarrow", min_version="0.15.0")
    def test_additional_extension_arrays(self, pa):
        # test additional ExtensionArrays that are supported through the
        # __arrow_array__ protocol
        df = pd.DataFrame(
            {
                "a": pd.Series([1, 2, 3], dtype="Int64"),
                "b": pd.Series(["a", None, "c"], dtype="string"),
            }
        )
        if LooseVersion(pyarrow.__version__) >= LooseVersion("0.15.1.dev"):
            expected = df
        else:
            # de-serialized as plain int / object
            expected = df.assign(a=df.a.astype("int64"), b=df.b.astype("object"))
>       check_round_trip(df, pa, expected=expected)

pandas/tests/io/test_parquet.py:522: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
pandas/tests/io/test_parquet.py:175: in check_round_trip
    compare(repeat)
pandas/tests/io/test_parquet.py:167: in compare
    df.to_parquet(path, **write_kwargs)
pandas/core/frame.py:2061: in to_parquet
    **kwargs,
pandas/io/parquet.py:255: in to_parquet
    **kwargs,
pandas/io/parquet.py:101: in write
    table = self.api.Table.from_pandas(df, **from_pandas_kwargs)
pyarrow/table.pxi:1057: in pyarrow.lib.Table.from_pandas
    ???
/usr/local/lib/python3.7/site-packages/pyarrow/pandas_compat.py:555: in dataframe_to_arrays
    for c, f in zip(columns_to_convert, convert_fields)]
/usr/local/lib/python3.7/site-packages/pyarrow/pandas_compat.py:555: in <listcomp>
    for c, f in zip(columns_to_convert, convert_fields)]
/usr/local/lib/python3.7/site-packages/pyarrow/pandas_compat.py:546: in convert_column
    raise e
/usr/local/lib/python3.7/site-packages/pyarrow/pandas_compat.py:540: in convert_column
    result = pa.array(col, type=type_, from_pandas=True, safe=safe)
pyarrow/array.pxi:196: in pyarrow.lib.array
    ???
pyarrow/array.pxi:92: in pyarrow.lib._handle_arrow_array_protocol
    ???
pandas/core/arrays/string_.py:200: in __arrow_array__
    return pa.array(self._ndarray, type=type, from_pandas=True)
pyarrow/array.pxi:207: in pyarrow.lib.array
    ???
pyarrow/array.pxi:78: in pyarrow.lib._ndarray_to_array
    ???
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

>   ???
E   pyarrow.lib.ArrowTypeError: ("Expected a string or bytes object, got a 'NAType' object", 'Conversion failed for column b with type string')
```