Allow null ObjectMapper in Jackson2ObjectMapperFactoryBean.getObjectType()

Issue: SPR-11785

spring-web/src/main/java/org/springframework/http/converter/json/Jackson2ObjectMapperFactoryBean.java

@@ -486,8 +486,7 @@ public class Jackson2ObjectMapperFactoryBean implements FactoryBean<ObjectMapper
 
 	@Override
 	public Class<?> getObjectType() {
-		Assert.notNull(this.objectMapper, "ObjectMapper must not be null");
-		return this.objectMapper.getClass();
+		return (this.objectMapper != null) ? this.objectMapper.getClass() : null;
 	}
 
 	@Override

spring-web/src/test/java/org/springframework/http/converter/json/Jackson2ObjectMapperFactoryBeanTests.java

@@ -182,6 +182,11 @@ public class Jackson2ObjectMapperFactoryBeanTests {
 		assertEquals(ObjectMapper.class, this.factory.getObjectType());
 	}
 
+	@Test
+	public void undefinedObjectType() {
+		assertEquals(null, this.factory.getObjectType());
+	}
+
 	private static SerializerFactoryConfig getSerializerFactoryConfig(ObjectMapper objectMapper) {
 		return ((BasicSerializerFactory) objectMapper.getSerializerFactory()).getFactoryConfig();
 	}