BUG: Cannot index into DataFrame with Nullable Integer as index 
#### Code Sample
```python
import pandas as pd
import numpy as np

df = pd.DataFrame(data=np.array([[1,2,3,4],[5,6,7,8],[1,2,np.nan,np.nan]]).T,columns=('a','b','c'),dtype="Int64")
df2 = df.set_index('c')

df2.loc[1]
```

#### Problem description
When there is more than one missing value in the dataframe index, indexing into the dataframe results in 
```
TypeError: Cannot convert bool to numpy.ndarray
```




If only one NaN index exists, this works:
```python
df = pd.DataFrame(data=np.array([[1,2,3,4],[5,6,7,8],[1,2,3,np.nan]]).T,columns=('a','b','c'),dtype="Int64")
df2 = df.set_index('c')
df2.loc[1]
```
```
a    1
b    5
Name: 1, dtype: Int64
```

#### Expected Output
I Expect `df2.loc[1]` to return

```
a    1
b    5
Name: 1, dtype: Int64
```


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

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.6.9.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.3.0-53-generic
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.0.3
numpy            : 1.18.2
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 20.1.1
setuptools       : 46.1.3.post20200330
Cython           : 0.29.15
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : 4.2.1
html5lib         : 0.999999999
pymysql          : None
psycopg2         : None
jinja2           : 2.11.1
IPython          : 7.13.0
pandas_datareader: None
bs4              : 4.6.0
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : 4.2.1
matplotlib       : 3.1.2
numexpr          : 2.6.4
odfpy            : None
openpyxl         : 3.0.3
pandas_gbq       : None
pyarrow          : None
pytables         : None
pytest           : None
pyxlsb           : None
s3fs             : None
scipy            : 1.4.1
sqlalchemy       : None
tables           : 3.4.2
tabulate         : 0.8.7
xarray           : None
xlrd             : 1.2.0
xlwt             : None
xlsxwriter       : None
numba            : 0.48.0
​
</details>
