REGR: Series repr of object Index with bools and NaN is wrong
#### Code Sample, a copy-pastable example if possible

```
import pandas as pd
# a series with booleans and nan
pd.Series([False,True,True,pd.NA]).value_counts(dropna=False)
True     2
False    1
True     1
dtype: int64

# check the actual index of the value_counts result
pd.Series([False,True,True,pd.NA]).value_counts(dropna=False).index
Index([True, False, nan], dtype='object')

# a similar Serie, with ints instead of nans, seems to work ok
pd.Series([0,1,1,pd.NA]).value_counts(dropna=False)
1.0    2
0.0    1
NaN    1
dtype: int64
```
#### Problem description
As shown in the example, the __repr__ of the value_counts result is apparently wrong when booleans are in the serie. It should report nan as a possible value, instead it maps it to True.
Note that this is limited to series with boolean. A similar example with ints works ok.


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

<details>
INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.1.final.0
python-bits      : 64
OS               : Linux
OS-release       : 4.15.0-76-generic
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.0.1
numpy            : 1.18.1
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 20.0.2
setuptools       : 45.2.0
Cython           : None
pytest           : 5.3.5
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : 2.8.4 (dt dec pq3 ext lo64)
jinja2           : None
IPython          : 7.12.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : None
matplotlib       : 3.1.3
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
pytest           : 5.3.5
pyxlsb           : None
s3fs             : None
scipy            : 1.4.1
sqlalchemy       : 1.3.13
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None
numba            : None

</details>
