Add @Nullable annotations to join and formatted.

For `join`, compare jspecify/jdk#111.

I might have done `formatted` during a mega-merge? I don't remember
offhand. Anyway, it makes sense for it to match `format`.

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

@@ -3379,7 +3379,7 @@ public String replace(@GuardSatisfied CharSequence target, @GuardSatisfied CharS
      */
     @SideEffectFree
     @StaticallyExecutable
-    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++) {
@@ -3475,7 +3475,7 @@ static String join(String prefix, String suffix, String delimiter, String[] elem
     @SideEffectFree
     @StaticallyExecutable
     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();
@@ -4393,7 +4393,7 @@ public static String format(@GuardSatisfied @Nullable Locale l, String format, @
      *
      */
     @SideEffectFree
-    public String formatted(Object... args) {
+    public String formatted(@Nullable Object... args) {
         return new Formatter().format(this, args).toString();
     }