BUG: Period resample with length=0 doesn't set freq
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

given freq is not applied if data is empty.

```
s = pd.Series(index=pd.PeriodIndex([], freq='D'))
s.resample('M').mean()
# Series([], Freq: D, dtype: float64)
```
#### Expected

```
s.resample('M').mean()
# Series([], Freq: M, dtype: float64)
```
