BUG 0.20.0rc1: Empty series for `var`/`std` for frame with non-numeric columns
```python
In [1]: import pandas as pd

In [2]: pd.__version__
Out[2]: '0.20.0rc1'

In [3]: df = pd.DataFrame({'int': [1, 2, 3, 4],
   ...:                    'float': [1., 2., 3., 4.],
   ...:                    'str': ['a', 'b', 'c', 'd']})

In [4]: df.mean()
Out[4]:
float    2.5
int      2.5
dtype: float64

In [5]: df.std()
Out[5]:
Series([], dtype: float64)

In [6]: df.var()
Out[6]:
Series([], dtype: float64)
```