Add required @nonnull annotation

Author: wmdietl

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

@@ -437,7 +437,7 @@ public Stream<T> stream() {
     @CFComment({"optional: orElseThrow(Supplier) does not throw NoSuchElementException, so its receiver is @MaybePresent.",
                 "Contrast with orElseThrow(), defined just above, whose receiver is @Present."})
     @EnsuresQualifier(expression = "this", qualifier = Present.class)
-    public <X extends Throwable> T orElseThrow(Supplier<? extends X> exceptionSupplier) throws X {
+    public <X extends Throwable> @NonNull T orElseThrow(Supplier<? extends X> exceptionSupplier) throws X {
         if (value != null) {
             return value;
         } else {