Series.min(skipna=True) includes nan/inf?
```
In[31]: s2 = pd.Series(["a","z",np.nan])
In[32]: s2.max()
Out[32]: 'z'
In[33]: s2.min()
Out[33]: inf
In[34]: s2.min(skipna=True)
Out[34]: inf
```

Reading the docs I get the impression that it should return "a" in both cases (as skipna defaults to True).

If so, #7217 also needs to change.
