fix a few issues when using shortcuts on firefox

Fixed https://bugzilla.redhat.com/show_bug.cgi?id=1150373#c9:
- tab on untranslated entry will trigger a save
- tab on last entry will trigger an error

Does something happen between line 60 and here that would change the return value of `needToSavePhrase(phrase)`, such as the save request? If not, this check can be omitted.