Add missing @Contract annotation to ObjectUtils#isEmpty

Closes gh-33984

Author: sdeleuze

spring-core/src/main/java/org/springframework/util/ObjectUtils.java

@@ -138,6 +138,7 @@ public static boolean isEmpty(@Nullable Object[] array) {
 	 * @see CollectionUtils#isEmpty(java.util.Collection)
 	 * @see CollectionUtils#isEmpty(java.util.Map)
 	 */
+	@Contract("null -> true")
 	public static boolean isEmpty(@Nullable Object obj) {
 		if (obj == null) {
 			return true;