BUG: Series.groupby returns an error if index is float and size is >= 1000000
- [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

```
# Imports
import numpy as np
import pandas as pd

# Values
nsteps = 5*10**6
sv = np.random.normal(loc=100.0, scale=1.0, size=nsteps+1)

# Series with int index
se = pd.Series(sv)

# Series with float index
flind = np.arange(nsteps+1)/nsteps
sef = pd.Series(sv, index=flind)

# Group with int index works
seg = se.groupby(level=0).last()

# Group with float index and size<1000000 works
sefg = sef.iloc[:999999].groupby(level=0).last()

# Group with float index and size>=1000000 doesn't work
sefg2 = sef.iloc[:1000000].groupby(level=0).last()
```

#### Problem description

Series.groupby returns an error if index is float and size is >= 1000000 (no problem with this size if index is int):

File "pandas/_libs/index.pyx", line 345, in pandas._libs.index._bin_search
TypeError: '<' not supported between instances of 'float' and 'NoneType'
![Screenshot 2020-08-18 at 13 51 08](https://user-images.githubusercontent.com/14150204/90515006-e5aed280-e159-11ea-8b05-4a56aa568aa6.png)


#### Expected Output

sefg2 should be similar to sefg (with one additional row)
![Screenshot 2020-08-18 at 13 39 59](https://user-images.githubusercontent.com/14150204/90515138-1abb2500-e15a-11ea-85f4-60b72d389ebe.png)
![Screenshot 2020-08-18 at 13 39 23](https://user-images.githubusercontent.com/14150204/90515165-26a6e700-e15a-11ea-96fb-6ab7c749b674.png)


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

<details>

INSTALLED VERSIONS
------------------
commit           : d9fff2792bf16178d4e450fe7384244e50635733
python           : 3.7.7.final.0
python-bits      : 64
OS               : Darwin
OS-release       : 19.6.0
Version          : Darwin Kernel Version 19.6.0: Thu Jun 18 20:49:00 PDT 2020; root:xnu-6153.141.1~1/RELEASE_X86_64
machine          : x86_64
processor        : i386
byteorder        : little
LC_ALL           : en_US.UTF-8
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.1.0
numpy            : 1.19.0
pytz             : 2020.1
dateutil         : 2.8.1
pip              : 20.1.1
setuptools       : 49.1.0.post20200704
Cython           : None
pytest           : 5.4.3
hypothesis       : None
sphinx           : 3.1.1
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.11.2
IPython          : 7.17.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fsspec           : None
fastparquet      : None
gcsfs            : None
matplotlib       : 3.3.0
numexpr          : 2.7.1
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
pyxlsb           : None
s3fs             : None
scipy            : 1.5.2
sqlalchemy       : None
tables           : 3.6.1
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
numba            : 0.50.1

</details>
