Resolve warning

Author: nakamura-to

doma-core/src/test/java/org/seasar/doma/jdbc/query/UpsertAssemblerContextTest.java

@@ -319,7 +319,7 @@ public Optional<Class<?>> getDomainClass() {
       }
 
       @Override
-      public Wrapper getWrapper() {
+      public Wrapper<Object> getWrapper() {
         return null;
       }
     };