Fix reference leak in ReifyTimestampAndWindowsParDoFnFactory (#36968)

Author: Abacn

runners/google-cloud-dataflow-java/worker/src/main/java/org/apache/beam/runners/dataflow/worker/ReifyTimestampAndWindowsParDoFnFactory.java

@@ -87,9 +87,13 @@ public void processElement(Object untypedElem) throws Exception {
     public void processTimers() {}
 
     @Override
-    public void finishBundle() throws Exception {}
+    public void finishBundle() throws Exception {
+      this.receiver = null;
+    }
 
     @Override
-    public void abort() throws Exception {}
+    public void abort() throws Exception {
+      this.receiver = null;
+    }
   }
 }