8335637: Add explicit non-null return value expectations to Object.toString()

Reviewed-by: jpai
Backport-of: 66db71563c3ebd715a1192a9b399b618d7bdb8d0

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

@@ -237,6 +237,10 @@ public class Object {
 
     /**
      * {@return a string representation of the object}
+     *
+     * Satisfying this method's contract implies a non-{@code null}
+     * result must be returned.
+     *
      * @apiNote
      * In general, the
      * {@code toString} method returns a string that