[ci] Add missing serial version uid as 1L

Author: hazendaz

src/main/java/org/apache/ibatis/cache/CacheKey.java

@@ -31,6 +31,8 @@ public class CacheKey implements Cloneable, Serializable {
 
   public static final CacheKey NULL_CACHE_KEY = new CacheKey() {
 
+    private static final long serialVersionUID = 1L;
+
     @Override
     public void update(Object object) {
       throw new CacheException("Not allowed to update a null cache key instance.");