- Update the ShutdownDisabledTest.java file to fix the assertion check for shutdown callback

Author: Pluto-syd

log4j-core-test/src/test/java/org/apache/logging/log4j/core/ShutdownDisabledTest.java

@@ -35,6 +35,6 @@ void testShutdownFlag(final Configuration config, final LoggerContext ctx) throw
         Field shutdownCallback = LoggerContext.class.getDeclaredField("shutdownCallback");
         Object fieldValue = ReflectionUtil.getFieldValue(shutdownCallback, ctx);
         assertFalse(config.isShutdownHookEnabled(), "Shutdown hook is enabled");
-        assertNull(fieldValue, "Shutdown callback is not null");
+        assertNull(fieldValue, "expected: <null> but was: " + fieldValue);
     }
 }