Minimum of ordered categorical data in Panda DataFrames
I have a Pandas DataFrame with one Serie containing ordered Categorical data. Some value of this Serie may be missing (NaN). I want to get the minimum without taking into account NaNs but I obtained strange results ...

#### Code:
```python
raw_cat = pd.Categorical(["a", "b", "c", "a"],
                         categories=["b", "c", "d"],
                         ordered=True)
s = pd.Series(raw_cat)
raw_cat.min(numeric_only=True), s.min(numeric_only=True)
```
#### Output:
```python
('b', nan)
```
#### Expected utput:
```python
('b', 'b')
```

I am getting the desired output when running this code with pandas 0.23.4 but not with pandas 0.24.0 and above.

Is this an issue or a misunderstanding? Thank you for your help.