public int nextCodePoint(int index) {
        int next = nextIndex(index);
        return next >= s.length() ? '\u0000' : s.codePointAt(next);
    }
Returns the code point at the next position after the given index,
or \u0000 if the index is at the last code point
vespajlib/src/main/java/com/yahoo/text/UnicodeString.java