BUG: groupby-nunique modifies null values
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd
import numpy as np
df = pd.DataFrame({"GROUP": 0, "VALUE": [1.0, np.nan]})
df.groupby("GROUP")["VALUE"].nunique()
print(df)
```
#### Problem description
Original dataframe is modified:
```
   GROUP         VALUE
0      0  1.000000e+00
1      0 -9.223372e+18
```

Issue seems to have been introduced in version 1.0.0, 0.25.3 works as expected.

#### Expected Output
Original dataframe should not be modified.

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

<details>
INSTALLED VERSIONS
------------------
commit           : None
python           : 3.6.6.final.0
python-bits      : 64
OS               : Linux
OS-release       : 4.9.87-linuxkit-aufs
machine          : x86_64
processor        :
byteorder        : little
LC_ALL           : en_US.UTF-8
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.0.1
numpy            : 1.17.2
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 19.2.3
setuptools       : 41.2.0
Cython           : 0.29.13
pytest           : 5.2.1
hypothesis       : 4.23.0
sphinx           : 1.7.9
blosc            : None
feather          : None
xlsxwriter       : 1.2.2
lxml.etree       : 4.4.1
html5lib         : 1.0.1
pymysql          : None
psycopg2         : 2.8.2 (dt dec pq3 ext lo64)
jinja2           : 2.10.3
IPython          : 7.9.0
pandas_datareader: None
bs4              : None
bottleneck       : 1.2.1
fastparquet      : None
gcsfs            : None
lxml.etree       : 4.4.1
matplotlib       : 3.1.1
numexpr          : 2.7.0
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : 0.13.0
pytables         : None
pytest           : 5.2.1
pyxlsb           : None
s3fs             : None
scipy            : 1.2.1
sqlalchemy       : 1.3.11
tables           : None
tabulate         : 0.8.5
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : 1.2.2
numba            : 0.45.1
</details>
