OZG-6544 fix unit test

Sub task: OZG-7191

alfa-service/src/test/java/de/ozgcloud/alfa/postfach/PostfachMailModelAssemblerTest.java

@@ -45,6 +45,7 @@ import org.springframework.hateoas.mediatype.hal.HalModelBuilder;
 import org.springframework.web.util.UriComponentsBuilder;
 
 import com.fasterxml.jackson.databind.ObjectMapper;
+import com.fasterxml.jackson.datatype.jdk8.Jdk8Module;
 import com.fasterxml.jackson.datatype.jsr310.JavaTimeModule;
 
 import de.ozgcloud.alfa.common.user.UserId;
@@ -376,7 +377,7 @@ class PostfachMailModelAssemblerTest {
 		@Test
 		@SneakyThrows
 		void shouldContainPostfachMails() {
-			var objectMapper = new ObjectMapper().registerModule(new JavaTimeModule());
+			var objectMapper = new ObjectMapper().registerModules(new JavaTimeModule(), new Jdk8Module());
 			doReturn(EntityModel.of(postfachMail)).when(modelAssembler).toModel(postfachMail);
 
 			var model = callModelAssembler();