BUG: Mixed DataFrame with Extension Array incorrect aggregation
Surprised to not get an aggregation for "b" with the Int64 dtype

```python
>>> df = pd.DataFrame([["a", 1]], columns=list("ab"))
>>> df.sum()
a    a
b    1
dtype: object
>>> df.astype({"b": "Int64"}).sum()
a    a
dtype: object
```