Remove unused test assertion import

Clean up `EventPublishingConfigurationPropertiesBeanPropertyChangedListenerTest` by removing an unused `assertNull` import.

Author: mercyblitz

microsphere-spring-boot-core/src/test/java/io/microsphere/spring/boot/context/properties/bind/EventPublishingConfigurationPropertiesBeanPropertyChangedListenerTest.java

@@ -55,7 +55,6 @@
 import static org.junit.jupiter.api.Assertions.assertArrayEquals;
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertNotNull;
-import static org.junit.jupiter.api.Assertions.assertNull;
 import static org.junit.jupiter.api.Assertions.assertSame;
 import static org.mockito.Mockito.mock;
 import static org.mockito.Mockito.when;