[Java] Fix serializing issues of RaySerializer (#4887)

* Fix

* Address comment.

java/runtime/pom.xml

@@ -54,7 +54,7 @@
 <dependency>
   <groupId>de.ruedigermoeller</groupId>
   <artifactId>fst</artifactId>
-  <version>2.47</version>
+  <version>2.57</version>
 </dependency>
 <dependency>
   <groupId>org.apache.commons</groupId>

java/runtime/src/main/java/org/ray/runtime/RayPyActorImpl.java

@@ -20,7 +20,9 @@ public class RayPyActorImpl extends RayActorImpl implements RayPyActor {
    */
   private String className;
 
-  private RayPyActorImpl() {}
+  // Note that this empty constructor must be public
+  // since it'll be needed when deserializing.
+  public RayPyActorImpl() {}
 
   public RayPyActorImpl(UniqueId id, String moduleName, String className) {
     super(id);