Annotate Cleaner. (#142)

Author: cpovirk

src/java.base/share/classes/java/lang/ref/Cleaner.java

@@ -30,6 +30,8 @@
 import java.util.Objects;
 import java.util.concurrent.ThreadFactory;
 import java.util.function.Function;
+import org.jspecify.annotations.NullMarked;
+import org.jspecify.annotations.Nullable;
 
 /**
  * {@code Cleaner} manages a set of object references and corresponding cleaning actions.
@@ -132,6 +134,7 @@
  * All cleaning actions registered to a cleaner should be mutually compatible.
  * @since 9
  */
+@NullMarked
 public final class Cleaner {
 
     /**