makes user_id field nullable

If a user attempts to sign in with an account that does not exists, the exception below is thrown, because Sentry attempts to update the throttle table with a null user_id. This makes the user_id field nullable.

```
Illuminate \ Database \ QueryException
SQLSTATE[HY000]: General error: 1364 Field 'user_id' doesn't have a default value (SQL: insert into `throttle` (`type`, `updated_at`, `created_at`) values (global, 2014-03-27 08:55:12, 2014-03-27 08:55:12))
```