Resampling and counting empty Series does not have a correct dtype
#### Code Sample, a copy-pastable example if possible

Short examples:

```python
import pandas as pd

a = pd.Series([], dtype="datetime64[ns]", index=pd.DatetimeIndex([]))
print(a.resample("D").size().dtype) # => actual datetime64[ns], expected int64
```
Longer example:

```python
import pandas as pd

t = pd.to_datetime(0)

x = pd.Series([t], index=[t])
print(x.resample("D").size().dtype) # => int64 (OK)

y = x.iloc[0:1]
print(y.resample("D").size().dtype) # => int64 (OK)

z = x.iloc[0:0]
print(z.resample("D").size().dtype) # => actual datetime64[ns], expected int64
```

#### Problem description

When `.resample().size()` an empty dataframe, the resulting dtype is the dtype of the initial Series. It should be an integer.

This means that this kind of things fail if the input `Series` happens to be empty:

```python
monthy_rate = (x.resampe("D").size() > 0).resample("M").mean()
```

#### Expected Output

<pre>
int64
int64
int64
</pre>

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

<details>
INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.4.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.2.0-2-amd64
machine          : x86_64
processor        : 
byteorder        : little
LC_ALL           : None
LANG             : fr_FR.utf8
LOCALE           : fr_FR.UTF-8

pandas           : 0.25.1
numpy            : 1.17.2
pytz             : 2019.2
dateutil         : 2.8.0
pip              : 18.1
setuptools       : 40.8.0
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : None
IPython          : None
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : None
matplotlib       : None
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
s3fs             : None
scipy            : None
sqlalchemy       : None
tables           : None
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None
</details>
