pd.Series.sum() returns nan for Int64 with pd.NA
#### Code Sample, a copy-pastable example if possible

I'm not sure this is a bug or intended behavior. If intended, I'm glad to be pointed to how API design was discussed and decided.

On `1.0.0 rc0`, `pd.Series.sum()`returns `nan` when using `skipna=False` for `'Int64'` with `pd.NA`. 

```python
>>> pd.Series([1, 2, pd.NA]).sum(skipna=False)
<NA>
>>> pd.Series([1, 2, pd.NA], dtype='Int64').sum(skipna=False)
nan

>>> pd.Series([1, 2, np.nan]).sum(skipna=False)
nan
>>> pd.Series([1, 2, np.nan], dtype='Int64').sum(skipna=False)
nan

>>> pd.Series([1, 2, None]).sum(skipna=False)
nan
>>> pd.Series([1, 2, None], dtype='Int64').sum(skipna=False)
nan
```

#### Problem description

I understand the second and third example returns `nan` for backward compatibility, but the first example of `dtype='Int64'` seems natural to return `pd.NA`.

#### Expected Output

```python
>>> pd.Series([1, 2, pd.NA], dtype='Int64').sum(skipna=False)
<NA>

# When breaking backward compatibility is allowed, this is also expected (on and after pandas 2.0.0?)
>>> pd.Series([1, 2, np.nan], dtype='Int64').sum(skipna=False)
<NA>
>>> pd.Series([1, 2, None], dtype='Int64').sum(skipna=False)
<NA>
```


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

<details>
>>> pd.show_versions()

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.6.final.0
python-bits      : 64
OS               : Linux
OS-release       : 4.9.184-linuxkit
machine          : x86_64
processor        : 
byteorder        : little
LC_ALL           : None
LANG             : C.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.0.0rc0
numpy            : 1.18.1
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 19.3.1
setuptools       : 44.0.0
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : None
IPython          : None
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : None
matplotlib       : 3.1.2
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
pytest           : None
s3fs             : None
scipy            : None
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None
numba            : None


```Dockerfile
# Dockerfile
FROM python:3.7.6
WORKDIR /home
RUN pip install pandas==1.0.0rc0
CMD ["/bin/bash"]
```
</details>
