BUG: where(...) unexpectedly casts None to np.NA for series with pd.Int64Dtype
- [x] I have checked that this issue has not already been reported.
- [x] I have confirmed this bug exists on the latest version of pandas.
- [ ] (optional) I have confirmed this bug exists on the master branch of pandas.

---
#### Problem description
When trying to replace all `pd.NA` values with `None` using `where(...)` on a `pd.Series` with dtype `pd.Int64Dtype`, `None` values are automatically coerced to `pd.NA` in a way inconsistent with other dtypes. Additionally, the `try_cast` flag doesn't control this behavior.

#### Expected Output
I expect that when `try_cast=False`, `None` will not be coerced into `np.NA`, and the dtype of the series will instead be cast into `object` to accommodate the newly introduced `None` value, as occurs for other dtypes.

#### Example
In the following:
```python
s = pd.Series([1, 2, 3, pd.NA], dtype=pd.Int64Dtype())
s.where(pd.notnull(s), None)
```
the `None` I'm trying to replace `pd.NA` with is automatically cast back into `pd.NA`. This occurs even though `try_cast` is set to `False` by default, which I would expect to control this casting behavior.
```
0       1
1       2
2       3
3    <NA>
dtype: Int64
```
This is in contrast to the behavior of other dtypes, like numpy's float dtype:
```python
s = pd.Series([1, 2, 3, np.nan], dtype=np.dtype("float64"))
s.where(pd.notnull(s), None)
```
which outputs the series with the dtype coerced to `object` after incorporating `None`:
```
0       1
1       2
2       3
3    None
dtype: object
```

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.8.2.final.0
python-bits      : 64
OS               : Darwin
OS-release       : 18.7.0
machine          : x86_64
processor        : i386
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : en_US.UTF-8
pandas           : 1.0.4
numpy            : 1.18.4
pytz             : 2020.1
dateutil         : 2.8.1
pip              : 19.2.3
setuptools       : 41.2.0
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.11.1
IPython          : None
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : None
matplotlib       : None
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
pytest           : None
pyxlsb           : None
s3fs             : None
scipy            : None
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None
numba            : None


</details>
