Fix undefined method empty? for nil:NilClass when user was not found

If we pass wrong data in get_user_by_id and get_user_by_username we have this error.

```
Failure/Error: puts client.get_user_by_id('gotrunko')
     NoMethodError:
       undefined method `empty?' for nil:NilClass
     # ./lib/grooveshark/client.rb:28:in `get_user_by_id'
```