[Whisper] fix all issues with unk token
# What does this PR do?
Previously, all OOV ( and thus timestamp tokens) outputed by the model are decoded to `<|endoftext|>` by the `xxx.en` whisper models. This does not happen with the multilingual model only because I added `""` to the vocabulary, and the `unk_token_id` is the same `""`. But this does not really make sense. 
As the default behavior for Whisper is just to outptu `""` for any OOV, now the `_convert_id_to_token` function does not use a `unk_token`. 

This will fix the inconsistency, and will help for the whisper refactoring. 

_The documentation is not available anymore as the PR was closed or merged._