ApplicationListenerMethodAdapter: gracefully handle beans which are actually NullBean

Currently, if you have an optional event listener (via a @Bean method returning `null`)
this causes the event multicaster to explode violently.  Now, we just safely skip it.

spring-context/src/test/java/org/springframework/context/event/AnnotationDrivenEventListenerTests.java

@@ -16,6 +16,8 @@
 
 package org.springframework.context.event;
 
+import java.io.Closeable;
+import java.io.IOException;
 import java.lang.annotation.ElementType;
 import java.lang.annotation.Retention;
 import java.lang.annotation.RetentionPolicy;
@@ -24,12 +26,14 @@ import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.LinkedHashSet;
 import java.util.List;
+import java.util.Optional;
 import java.util.Set;
 import java.util.concurrent.CompletableFuture;
 import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.TimeUnit;
 
 import javax.annotation.PostConstruct;
+import javax.inject.Inject;
 
 import org.junit.jupiter.api.AfterEach;
 import org.junit.jupiter.api.Disabled;
@@ -622,6 +626,12 @@ public class AnnotationDrivenEventListenerTests {
 		assertThat(listener.order).contains("first", "second", "third");
 	}
 
+	@Test
+	public void missingBeanDoesntCrash() {
+	    load(MissingEventListener.class);
+	    context.getBean(UseMissingEventListener.class);
+	    context.getBean(ApplicationEventMulticaster.class).multicastEvent(new TestEvent(this));
+	}
 
 	private void load(Class<?>... classes) {
 		List<Class<?>> allClasses = new ArrayList<>();
@@ -1076,4 +1086,32 @@ public class AnnotationDrivenEventListenerTests {
 		}
 	}
 
+	@Configuration
+	@Import({UseMissingEventListener.class})
+	public static class MissingEventListener {
+	    @Bean
+	    public MyEventListener missing() {
+	        return null;
+	    }
+	}
+
+	@Component
+	public static class MyEventListener implements Closeable {
+	    @EventListener
+	    public void hear(TestEvent e) {
+	        throw new AssertionError();
+	    }
+
+	    @Override
+	    public void close() throws IOException {}
+	}
+
+	public static class UseMissingEventListener {
+	    @Inject
+	    public UseMissingEventListener(Optional<MyEventListener> notHere) {
+	        if (notHere.isPresent()) {
+	            throw new AssertionError();
+	        }
+	    }
+	}
 }