Minor: set task to null at the end of shouldWrapProducerFencedExceptionWithTaskMigragedExceptionInSuspendWhenCommitting (#5534)

Reviewers: Matthias J. Sax <matthias@confluent.io>, Guozhang Wang <guozhang@confluent.io>

streams/src/test/java/org/apache/kafka/streams/processor/internals/StreamTaskTest.java

@@ -983,6 +983,7 @@ public class StreamTaskTest {
         } catch (final TaskMigratedException expected) {
             assertTrue(expected.getCause() instanceof ProducerFencedException);
         }
+        task = null;
 
         assertFalse(producer.transactionCommitted());
     }