Fix SingletonSupplier.ofNullable(T) nullability

See gh-35559

spring-core/src/main/java/org/springframework/util/function/SingletonSupplier.java

@@ -143,7 +143,7 @@ public class SingletonSupplier<T extends @Nullable Object> implements Supplier<T
 	 * @return the singleton supplier, or {@code null} if the instance was {@code null}
 	 */
 	@Contract("null -> null; !null -> !null")
-	public static <T extends @Nullable Object> @Nullable SingletonSupplier<T> ofNullable(T instance) {
+	public static <T extends @Nullable Object> @Nullable SingletonSupplier<T> ofNullable(@Nullable T instance) {
 		return (instance != null ? new SingletonSupplier<>(instance) : null);
 	}