Pandas Series: maximum recursion error when replacing empty lists
I have a pandas Series containing lists. I wanted to replace empty lists with NaN. My first approach was using .replace, but that unexpectedly gave me a maximum recursion error.

I am using pandas 0.21.1 on Python 3.4, but other users had the same problem on 0.23

```
import numpy as np
import pandas as 
ts = pd.Series([[1], [2, 3], [], [4]])
ts.replace([], np.nan)

RuntimeError: maximum recursion depth exceeded in comparison
```


I achieved the desired output using:

`ts[ts.apply(len) == 0] = np.nan`

but I don't understand the maximum recursion error