Fixed #7220 -- Allowed AbstractBaseUser.last_login to be null.

Maybe you can do the following:

```python
if user.last_login:
    login_timestamp = user.last_login.replace(microsecond=0, tzinfo=None)
else:
    login_timestamp = ''

value = (six.text_type(user.pk) + user.password +
    six.text_type(login_timestamp) + six.text_type(timestamp))
```

to avoid to have two conditions and to avoid inline ones.