Fix bcrypt hash error if `encrypted_password` is nil

Fixes the following error if a user attempts to authenticate to an account that doesn't have a password (for example, if it was created by omniauth):

```
BCrypt::Errors::InvalidHash - invalid hash:
  bcrypt (3.1.7) lib/bcrypt/password.rb:60:in `initialize'
  clearance (1.4.0) lib/clearance/password_strategies/bcrypt.rb:9:in `authenticated?'
  clearance (1.4.0) lib/clearance/user.rb:21:in `authenticate'
  clearance (1.4.0) lib/clearance/authentication.rb:18:in `authenticate'
```

Only this strategy is affected by this, the other strategies gracefully handle a nil `encrypted_password`.