Groupby creates emptu groups depending on base parameter 
#### Code Sample, a copy-pastable example if possible

```python
"Generate test dataframe"
case = 1
if case == 1:
    start = '2018-11-26 16:17:43.510000'
else:
    start = '2018-11-26 16:17:43.500000'

rng = pd.date_range(start, periods=10, freq='1S')
df = pd.DataFrame({'a':np.random.randn(len(rng)),'b':np.random.randn(len(rng))}, index=rng)

"Set interval and start time of the buckets"
interval = dt.timedelta(minutes=10)
t0 = df.index[0]
base = t0.minute + (t0.second +t0.microsecond/1e6)/60
groups = df.groupby(pd.Grouper(freq=interval, base=base))

print(groups.size())

```
#### Problem description

The code above generates either 6 or 7 groups depending if the dataframe starts at  '2018-11-26 16:17:43.500000' (case 1) or  '2018-11-26 16:17:43.510000' (case 2).

The correct output is clearly the one obtained in case 2. Case 1, instead, creates an empty group at the end of the dataframe. This can cause troubles with groupby.apply() if the applied function does not handle empty dataframes. 

#### Actual Output
2018-11-26 16:17:43.510    10
**2018-11-26 16:27:43.510      #0**
dtype: int64

#### Expected Output
2018-11-26 16:17:43.510    10
dtype: int64
#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.7.1.final.0
python-bits: 64
OS: Windows
OS-release: 7
machine: AMD64
processor: Intel64 Family 6 Model 78 Stepping 3, GenuineIntel
byteorder: little
LC_ALL: None
LANG: en
LOCALE: None.None
 
pandas: 0.23.4
pytest: 4.0.2
pip: 18.1
setuptools: 40.6.3
Cython: 0.29.2
numpy: 1.15.4
scipy: 1.1.0
pyarrow: None
xarray: None
IPython: 7.2.0
sphinx: 1.8.2
patsy: 0.5.1
dateutil: 2.7.5
pytz: 2018.7
blosc: None
bottleneck: 1.2.1
tables: 3.4.4
numexpr: 2.6.8
feather: None
matplotlib: 3.0.2
openpyxl: 2.5.12
xlrd: 1.2.0
xlwt: 1.3.0
xlsxwriter: 1.1.2
lxml: 4.2.5
bs4: 4.6.3
html5lib: 1.0.1
sqlalchemy: 1.2.15
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
</details>
