BUG: cannot write xml with nullable dtypes
### 

- [X] I have checked that this issue has not already been reported.

- [X] I have confirmed this bug exists on the [latest version](https://pandas.pydata.org/docs/whatsnew/index.html) of pandas.

- [X] I have confirmed this bug exists on the master branch of pandas.


### Reproducible Example

```python
import logging
import numpy as np
import pandas as pd
import pytest

logger = logging.getLogger("test")

class XmlTest:

    def test_to_xml_dtypes(self):
        dtypes = [
            pd.StringDtype(),
            pd.BooleanDtype(),
            pd.Float64Dtype(),
            pd.Float32Dtype(),
            pd.Int64Dtype(),
            pd.UInt64Dtype(),
            pd.Int32Dtype(),
            pd.UInt32Dtype(),
            pd.Int16Dtype(),
            pd.UInt16Dtype(),
            pd.Int8Dtype(),
            pd.UInt8Dtype(),
            pd.StringDtype(),
        ]
        data = [
            pd.Series({"abc": 1, "xyz": pd.NA}),
            pd.Series({"abc": pd.NA, "xyz": 0}),
        ]
        failed = {}
        for dtype in dtypes:
            df = pd.DataFrame(data).astype(dtype)
            try:
                df.to_xml()
            except TypeError as e:
                logger.error(dtype, exc_info=True)
                failed[str(dtype)] = str(e)
        assert failed == [], f"Failed on dtypes: {failed}"

if __name__ == "__main__":
    pytest.main()
```


### Issue Description

`to_xml` fails with nullable dtypes provided by Pandas like `Float64Dtype`. Example from a test run in CI; it fails on Ubuntu too.

### Expected Behavior

Output valid xml that can be read with `read_xml` to get approximately the same DataFrame.

### Installed Versions

<details>
INSTALLED VERSIONS
------------------
commit           : f00ed8f47020034e752baf0250483053340971b0
python           : 3.9.5.final.0
python-bits      : 64
OS               : Windows
OS-release       : 10
Version          : 10.0.19042
machine          : AMD64
processor        : Intel64 Family 6 Model 165 Stepping 2, GenuineIntel
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : English_United States.1252

pandas           : 1.3.0
numpy            : 1.21.2
pytz             : 2021.1
dateutil         : 2.8.1
pip              : 21.1.1
setuptools       : 52.0.0.post20210125
Cython           : None
pytest           : 6.2.4
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : 4.6.3
html5lib         : 1.1
pymysql          : 1.0.2
psycopg2         : None
jinja2           : None
IPython          : None
pandas_datareader: None
bs4              : 4.9.3
bottleneck       : None
fsspec           : 2021.07.0
fastparquet      : None
gcsfs            : None
matplotlib       : 3.4.2
numexpr          : None
odfpy            : None
openpyxl         : 3.0.7
pandas_gbq       : None
pyarrow          : 5.0.0
pyxlsb           : 1.0.8
s3fs             : None
scipy            : 1.7.0
sqlalchemy       : None
tables           : None
tabulate         : 0.8.9
xarray           : None
xlrd             : 2.0.1
xlwt             : None
numba            : None
None
</details>
