Fix potential NPE (#13505)

Author: AlbumenJ

dubbo-serialization/dubbo-serialization-hessian2/src/main/java/org/apache/dubbo/common/serialize/hessian2/Hessian2FactoryManager.java

@@ -23,6 +23,7 @@
 import org.apache.dubbo.common.utils.StringUtils;
 import org.apache.dubbo.rpc.model.FrameworkModel;
 
+import java.util.Objects;
 import java.util.concurrent.ConcurrentHashMap;
 
 import com.alibaba.com.caucho.hessian.io.SerializerFactory;
@@ -46,7 +47,7 @@ public Hessian2FactoryManager(FrameworkModel frameworkModel) {
 
     public SerializerFactory getSerializerFactory(ClassLoader classLoader) {
         SerializerFactory sticky = stickySerializerFactory;
-        if (sticky != null && sticky.getClassLoader().equals(classLoader)) {
+        if (sticky != null && Objects.equals(sticky.getClassLoader(), classLoader)) {
             return sticky;
         }