BUG: Categorical.from_codes should not warn on empty codes
Empty codes should not warn about float codes:

```python
>>> pd.Categorical.from_codes([], ['a', 'b', 'c']) 
C:\Users\TP\Miniconda3\envs\pandas-dev\Scripts\ipython:1: FutureWarning: float codes will be disallowed in the future and raise a ValueError
```
