Allow messageSupplier to be null.

The docs at best indirectly imply that it can be `null`: They have a
`@throws NullPointerException` clause that mentions only the case in
which `obj` is `null`, without saying anything similar about
`messageSupplier`.

But of course I'm actually going mostly off the implementation....

(Note that this PR is separate from
#104, which describes *another*
change that we could make to this parameter.)

src/java.base/share/classes/java/util/Objects.java

@@ -354,7 +354,7 @@ public static <T> T requireNonNullElseGet(@Nullable T obj, Supplier<? extends @N
      */
 
 
-    public static <T>  T requireNonNull(  @Nullable T obj,  Supplier<String> messageSupplier) {
+    public static <T>  T requireNonNull(  @Nullable T obj,  @Nullable Supplier<String> messageSupplier) {
         if (obj == null)
             throw new NullPointerException(messageSupplier == null ?
                                            null : messageSupplier.get());