Allow null CharSequence inputs to String.join. (#111)

Co-authored-by: Werner Dietl <[email protected]>

Author: cpovirk

src/java.base/share/classes/java/lang/String.java

@@ -3632,7 +3632,7 @@ public String[] split(String regex) {
      */
 
 
-    public static String join(CharSequence delimiter, CharSequence... elements) {
+    public static String join(CharSequence delimiter, @Nullable CharSequence... elements) {
         var delim = delimiter.toString();
         var elems = new String[elements.length];
         for (int i = 0; i < elements.length; i++) {
@@ -3728,7 +3728,7 @@ static String join(String prefix, String suffix, String delimiter, String[] elem
 
 
     public static String join(CharSequence delimiter,
-            Iterable<? extends CharSequence> elements) {
+            Iterable<? extends @Nullable CharSequence> elements) {
         Objects.requireNonNull(delimiter);
         Objects.requireNonNull(elements);
         var delim = delimiter.toString();