There is one overload of String.join method which have explicit requireNonNull checks - java.lang.String#join(java.lang.CharSequence, java.lang.Iterable<? extends java.lang.CharSequence>)

    public static String join(CharSequence delimiter,
            Iterable<? extends CharSequence> elements) {
        Objects.requireNonNull(delimiter);
        Objects.requireNonNull(elements);
        var delim = delimiter.toString();
        var elems = new String[8];
        int size = 0;
        for (CharSequence cs: elements) {

To squeeze a bit of performance and be consistent with other overloads we can remove explicit requireNonNull calls.
Modern java version have helpful NPE message, which are even better than empty message generated by Objects.requireNonNull