fix EloquentRepository getUserByIdentity query field when its value is empty

@Zizaco this is a pr related to the issue that @Doublemedia mentioned in #402 

    SQLSTATE[42S22]: Column not found: 1054 Unknown column 'username' in 'where clause' (SQL: select * from users where username = )

I reproduce the problem on a brand-new installation. 

In the `validateIsUnique`, the `username` field will be **set to empty** when username is optional. 

Then it'll call  `EloquentRepository->getUserByIdentity`, which will generate query something like this.

`User::where('username','=','')->get();`

And laravel will still look up the field `username` which doesn't exist so it cause the error.