BUG: Grouper.cov emits warning unlike DataFrame.cov
Like https://github.com/pandas-dev/pandas/issues/27470, another case where Grouper and DataFrame provide essentially the same method but diverge in behavior.

```python
import pandas as pd

df=pd.DataFrame(dict(a=[0,4,8],b=[2,7,12],c=[3,9,11]))
g=df.groupby([0,1,1])

# single row frame silently emits NaNs
df.iloc[[0]].cov()

# single row group emits multiple warnings
g.cov()
RuntimeWarning: Degrees of freedom <= 0 for slice 
```
