Make current_user method return nil when user not found

Fixes https://github.com/NoamB/sorcery/issues/493

@kirs what's your opinion on the issue above? The change from `nil` to `false` was made here https://github.com/NoamB/sorcery/commit/768eb976247b4da2c8a70d8660d7c8613c6545df to keep consistency (always return `false` instead of `nil` once and then `false`). I think it's better to always return `nil`, but feel free to just close the PR if you think otherwise