BUG: margin=True is affected by NaN in other columns in pivot_table
- [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.

- [x] (optional) I have confirmed this bug exists on the master branch of pandas.

---

**Note**: Please read [this guide](https://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports) detailing how to provide the necessary information for us to reproduce your bug.

#### Code Sample, a copy-pastable example

```python
>>> import pandas as pd
>>> data = {'A': [6, 7, None], 'B': [None, 5, 8], 'name': ['Alice', 'Alice', 'Alice']}
>>> df = pd.DataFrame(data)
>>> df.pivot_table(index='name', values=['A', 'B'], aggfunc=['min', 'max'], margins=True)
```

Output:
```python
       min       max     
         A    B    A    B
name                     
Alice  6.0  5.0  7.0  8.0
All    7.0  5.0  7.0  5.0
```

#### Problem description

The summary line 'All' shows min for A as 7.0 and max for B as 5.0 although min of A is 6.0 and max of B is 8.0

#### Expected Output
```python
       min       max     
         A    B    A    B
name                     
Alice  6.0  5.0  7.0  8.0
All    6.0  5.0  7.0  8.0
```

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

<details>

INSTALLED VERSIONS
------------------
commit           : 6bc8a49f2595daca6f6d0391020981d536747b23
python           : 3.8.2.final.0
python-bits      : 64
OS               : Darwin
OS-release       : 19.3.0
Version          : Darwin Kernel Version 19.3.0: Thu Jan  9 20:58:23 PST 2020; root:xnu-6153.81.5~1/RELEASE_X86_64
machine          : x86_64
processor        : i386
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.1.0.dev0+1225.g6bc8a49f2
numpy            : 1.18.1
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 20.0.2
setuptools       : 46.1.3.post20200325
Cython           : 0.29.16
pytest           : 5.4.1
hypothesis       : 5.8.0
sphinx           : 3.0.1
blosc            : None
feather          : None
xlsxwriter       : 1.2.8
lxml.etree       : 4.5.0
html5lib         : 1.0.1
pymysql          : None
psycopg2         : None
jinja2           : 2.11.1
IPython          : 7.13.0
pandas_datareader: None
bs4              : 4.9.0
bottleneck       : 1.3.2
fastparquet      : 0.3.3
gcsfs            : None
matplotlib       : 3.2.1
numexpr          : 2.7.1
odfpy            : None
openpyxl         : 3.0.3
pandas_gbq       : None
pyarrow          : 0.16.0
pytables         : None
pyxlsb           : None
s3fs             : 0.4.2
scipy            : 1.4.1
sqlalchemy       : 1.3.16
tables           : 3.6.1
tabulate         : 0.8.7
xarray           : 0.15.1
xlrd             : 1.2.0
xlwt             : 1.3.0
numba            : 0.48.0

</details>
