Add missing nullable annotation to ResponseEntity ok convenience method

Author: kevin0x90

spring-web/src/main/java/org/springframework/http/ResponseEntity.java

@@ -242,7 +242,7 @@ public static BodyBuilder ok() {
 	 * @return the created {@code ResponseEntity}
 	 * @since 4.1
 	 */
-	public static <T> ResponseEntity<T> ok(T body) {
+	public static <T> ResponseEntity<T> ok(@Nullable T body) {
 		return ok().body(body);
 	}