Change the way missing translation keys are detected

Hello,

Our translations are handled with Crowdin and one particularity with Crowdin is that, if a key wasn't translated yet and we sync them; Crowdin will return the key with an empty string instead of returning the file without the missing keys.

The thing is that, vue-i18n considers the empty string as an already-translated key, so it will uses it instead of fallbacking to the default version (english).

It would be great to override this default rule, so I can say that: "If my key is missing OR my key is empty, fallback to the default version".

An implementation suggestion would be a handler in the config that returns true or false if the key passed should be considered as missing?