BUG: concat series with NA in multiindex fail to combine rows of same index
- [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.

---

**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
# Your code here
import pandas as pd

names=['A1','A2']

idx1 = pd.MultiIndex.from_tuples([(1,1), (2, pd.NA)], names=names)
idx2 = pd.MultiIndex.from_tuples([(2, pd.NA), (3,3)], names=names)

s1 = pd.Series([11,12], index=idx1)
s2 = pd.Series([13,14], index=idx2)
s3 = pd.concat([s1, s2], axis=1)
print (s3)
# s3 fails to combine the two rows with index (2, NaN)
#            0     1
# A1 A2
# 1  1.0  11.0   NaN
# 2  NaN  12.0  13.0
#    NaN  12.0  13.0
# 3  3.0   NaN  14.0


# Workaround is to use merge after reset index
s1 = s1.reset_index()
s2 = s2.reset_index()
s3 = pd.merge(s1,s2,on=names,how='outer')
s3 = s3.set_index(names)
print (s3)
# Now the result is expected
# A1 A2
# 1  1.0  11.0   NaN
# 2  NaN  12.0  13.0
# 3  3.0   NaN  14.0
```

#### Problem description

[this should explain **why** the current behaviour is a problem and why the expected output is a better solution]

#### Expected Output

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

<details>

[paste the output of ``pd.show_versions()`` here leaving a blank line after the details tag]
INSTALLED VERSIONS
------------------
commit           : 2a7d3326dee660824a8433ffd01065f8ac37f7d6
python           : 3.8.5.final.0
python-bits      : 64
OS               : Darwin
OS-release       : 18.7.0
Version          : Darwin Kernel Version 18.7.0: Thu Jun 18 20:50:10 PDT 2020; root:xnu-4903.278.43~1/RELEASE_X86_64
machine          : x86_64
processor        : i386
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.1.2
numpy            : 1.19.1
pytz             : 2020.1
dateutil         : 2.8.1
pip              : 20.1.1
setuptools       : 47.1.0
Cython           : 0.29.21
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
fsspec           : None
fastparquet      : None
gcsfs            : None
matplotlib       : 3.3.1
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
pyxlsb           : None
s3fs             : None
scipy            : 1.5.2
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
numba            : None
</details>
