Fix XStream serializer issue in KafkaPublisher

Fix missed XStreamSerializer issue due to the Axon Framework 4.5.5 update

kafka/src/main/java/org/axonframework/extensions/kafka/eventhandling/producer/KafkaPublisher.java

@@ -240,8 +240,7 @@ public static class Builder<K, V> {
         @SuppressWarnings("unchecked")
         private KafkaMessageConverter<K, V> messageConverter =
                 (KafkaMessageConverter<K, V>) DefaultKafkaMessageConverter.builder()
-                                                                          .serializer(XStreamSerializer.builder()
-                                                                                                       .build())
+                                                                          .serializer(XStreamSerializer.defaultSerializer())
                                                                           .build();
         private MessageMonitor<? super EventMessage<?>> messageMonitor = NoOpMessageMonitor.instance();
         private String topic = DEFAULT_TOPIC;

kafka/src/test/java/org/axonframework/extensions/kafka/eventhandling/producer/KafkaPublisherBuilderTest.java

@@ -0,0 +1,22 @@
+package org.axonframework.extensions.kafka.eventhandling.producer;
+
+import org.junit.jupiter.api.*;
+import org.junit.jupiter.api.extension.*;
+import org.mockito.*;
+import org.mockito.junit.jupiter.*;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+@ExtendWith(MockitoExtension.class)
+class KafkaPublisherBuilderTest {
+
+    @Mock
+    private ProducerFactory<String, byte[]> producerFactory;
+
+    @Test
+    void testKafkaPublisherInitialisationShouldNotThrowException() {
+        assertDoesNotThrow(() -> KafkaPublisher.<String, byte[]>builder()
+                                               .producerFactory(producerFactory)
+                                               .build());
+    }
+}