fix($translate): if translation exists, use the translated string even if it's empty

Before:
$translate.instant() prints out the translation key if the string is empty. It doesn't, however, print out the translationNotFoundIndicators like it would do if the translation didn't exist at all.

After:
$translate.instant() always prints out the string you have set in the translated strings object, even if the string is empty (its length == 0).