Allow messageSupplier to be null. (#105)

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.)

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

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

@@ -351,7 +351,7 @@ public static <T> T requireNonNullElseGet(@Nullable T obj, Supplier<? extends @N
      * @throws NullPointerException if {@code obj} is {@code null}
      * @since 1.8
      */
-    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());