Space removed before () when used as default function argument

This
```python
import typing as t

def f1(x: t.Tuple[int, ...] = ()):
    pass
```

becomes this
```python
import typing as t

def f1(x: t.Tuple[int, ...] =()):
    pass
```


Operating system: Ubuntu
Python version: 3.6.3
Black version: 18.3a3
Does also happen on master: yes