BUG: Groupby sum, different result between 0.25.3 and 1.0.4
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- [X] I have confirmed this bug exists on the latest version of pandas.

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---
This is most likely a known problem, but I did not find an exact issue where to copy this behavior.
I ran into a problem between functionalities between 0.25.3 and 1.0.4.

Now:
```python
import pandas
df = pandas.DataFrame({"id": [1, 1], "value_1": [None, None], "value_2": [0, 0]})
df.loc[df["value_1"].isnull(), "value_1"] = 0
df.groupby("id").sum()
```

#### Problem description
In 1.0.4 the output is:
```python
    value_2
id         
1         0
```
Is this now the intended behaviour?
This is most likely because:
```python
df.dtypes
id          int64
value_1    object
value_2     int64
```

#### Expected Output
In 0.25.3
```python
    value_1  value_2
id                  
1         0        0
```
dtypes are
```python
df.dtypes
id          int64
value_1     int64
value_2     int64
```
#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.8.2.final.0
python-bits      : 64
OS               : Darwin
OS-release       : 19.5.0
machine          : x86_64
processor        : i386
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : en_GB.UTF-8
pandas           : 1.0.4
numpy            : 1.18.4
pytz             : 2020.1
dateutil         : 2.8.1
pip              : 20.1.1
setuptools       : 47.1.1
Cython           : None
pytest           : 5.3.2
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       : None
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
pytest           : 5.3.2
pyxlsb           : None
s3fs             : None
scipy            : None
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None
numba            : None

</details>
