This check was needed when `letterPosTop` was
`currentLetter?.offsetTop ?? previousLetter?.offsetTop ??
lastWordLetter?.offsetTop` where `previousLetter?.offsetTop` was defined
but 0 in extra letters in blind mode, so we got the value of zero
although in some fonts the real value (`lastWordLetter` value) is not 0.

However, now we don't use `previousLetter`, and `currentLetter` is
undefined in extra letters (blind mode or not), so we'll get the value
we want (`lastWordLetter` value) in extra letters.