rename_axis(None) now (0.24) does nothing. Previously it removed the axis name
#### Code Sample, a copy-pastable example if possible

In 0.23 `rename_axis` was able to remove the axis name like this:
```python
>>> s = pd.Series([1, 2, 3])
>>> s = s.rename_axis('asdf')
>>> s
asdf
0    1
1    2
2    3
dtype: int64

>>> s.rename_axis(None)
0    1
1    2
2    3
dtype: int64
```

In 0.24, the `mapper` parameter is defaulted to `None` so this does not work. The only way to remove an axis label is to use `del s.index.name` or `s.index.name = None`
#### Problem description

Seems like a regression. I have some code that relies on axis name removal with `s.rename_axis(None)`. Seems like there is no clean way to remove an axis name now.

#### Expected Output

`s.rename_axis(None)` should work as it did in 0.23.
