NDFrame.sum ignores min_count for object dtype
#### Code Sample, a copy-pastable example if possible

```python
>>> index = pd.MultiIndex.from_product([['a'], ['foo', 'bar']])
>>> s = pd.Series([np.nan] * 2, index=index)
>>> s.astype('O').sum(level=1, min_count=1)
foo    0
bar    0
dtype: int64

```
#### Problem description

Using sum with a multiindex and dtype set to object will force all answers to 0, and not `nan` as is expected. This is confusing, since pandas automatically coerces np.nan when this isn't the case, see below.

```python
>>> s = pd.Series([np.nan])
>>> s.astype('O').sum(min_count=1)
nan

```


<details>

[paste the output of ``pd.show_versions()`` here below this line]
INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.3.final.0
python-bits      : 64
OS               : Darwin
OS-release       : 18.7.0
machine          : x86_64
processor        : i386
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 0.25.1
numpy            : 1.16.3
pytz             : 2019.1
dateutil         : 2.8.0
pip              : 19.2.1
setuptools       : 41.0.1
Cython           : None
pytest           : 5.0.1
hypothesis       : None
sphinx           : 2.1.2
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : 4.4.0
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.10.1
IPython          : 7.5.0
pandas_datareader: None
bs4              : 4.8.0
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : 4.4.0
matplotlib       : None
numexpr          : None
odfpy            : None
openpyxl         : 2.6.2
pandas_gbq       : None
pyarrow          : None
pytables         : None
s3fs             : None
scipy            : None
sqlalchemy       : 1.3.6
tables           : None
xarray           : None
xlrd             : 1.2.0
xlwt             : None
xlsxwriter       : None
</details>
