BUG: DataFrameGroupBy.sum ignores min_count for boolean data type
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---

Behavior is from master:

```python
import pandas as pd

df = pd.DataFrame({"a": [1, 2], "b": pd.array([True, True])})
df.groupby("a").sum(min_count=2)
```
gives
```python
      b
a      
1  True
2  True
```
but expected output is
```python
      b
a      
1  <NA>
2  <NA>
```

It looks to me like there's an attempt to compute a Cythonized result which fails, after which point the min_count argument is forgotten.