Don't add separator when encoding is default with 'nondefault option

When `doom-modeline-buffer-encoding` is `'nondefault` and the current encoding _is_ the default encoding, it adds extra separators, and it's been bugging me :sweat_smile:. Before: <img width="476" height="63" alt="image" src="https://github.com/user-attachments/assets/2a63f585-3bd6-4d15-993e-c36ce2ab9fea" /> After: <img width="476" height="63" alt="image" src="https://github.com/user-attachments/assets/cc2036ef-1032-4546-968b-f688e052d2cc" />