BUG: groupby with empty object, categorical grouper, and dropna=False fails

```
df = DataFrame({'a': [1, 1, 2], 'b': [3, 4, 5]})
df['a'] = df['a'].astype('category')
df = df.iloc[:0]
gb = df.groupby('a', dropna=False, observed=True)
print(gb.sum())
```

gives `ValueError: attempt to get argmax of an empty sequence`