Polish "Avoid NPE when binder is closed before started event"

See gh-22141

spring-boot-project/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/metrics/web/tomcat/TomcatMetricsBinder.java

@@ -86,7 +86,7 @@ public class TomcatMetricsBinder implements ApplicationListener<ApplicationStart
 
 	@Override
 	public void destroy() {
-		if (null != this.tomcatMetrics) {
+		if (this.tomcatMetrics != null) {
 			this.tomcatMetrics.close();
 		}
 	}

spring-boot-project/spring-boot-actuator/src/test/java/org/springframework/boot/actuate/metrics/web/tomcat/TomcatMetricsBinderTests.java

@@ -0,0 +1,38 @@
+/*
+ * Copyright 2012-2020 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.actuate.metrics.web.tomcat;
+
+import io.micrometer.core.instrument.MeterRegistry;
+import org.junit.jupiter.api.Test;
+
+import static org.mockito.Mockito.mock;
+
+/**
+ * Tests for {@link TomcatMetricsBinder}.
+ *
+ * @author Andy Wilkinson
+ */
+class TomcatMetricsBinderTests {
+
+	private final MeterRegistry meterRegistry = mock(MeterRegistry.class);
+
+	@Test
+	void destroySucceedsWhenCalledBeforeApplicationHasStarted() {
+		new TomcatMetricsBinder(this.meterRegistry).destroy();
+	}
+
+}