Issue with default_value for List trait

I can't seem to set `default_value` of `List` trait to be `None`. When I try to set it the using the `default_value` argument in the constructor, the value is set to `[]`. After initialization I can set it to `None`. I am using `traitlets 4.3.1`. It works when the trait is an `Instance` of `List`.

```python
from traitlets import List, Instance
from ipywidgets import DOMWidget

class Test(DOMWidget):
arr = Instance(List, default_value=None, allow_none=True).tag(sync=True)
arr_2 = List(default_value=None, allow_none=True).tag(sync=True)

test_obj = Test()
print(test_obj.arr is None)
print(test_obj.arr_2 is None)
print(test_obj.arr_2)
```

Output:
True
False
[]