GH-1825: Fix Missing Nullable Annotation

Resolves #1825

Author: garyrussell

spring-kafka/src/main/java/org/springframework/kafka/config/KafkaListenerEndpointRegistry.java

@@ -41,6 +41,7 @@
 import org.springframework.kafka.listener.AbstractMessageListenerContainer;
 import org.springframework.kafka.listener.ListenerContainerRegistry;
 import org.springframework.kafka.listener.MessageListenerContainer;
+import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
 import org.springframework.util.StringUtils;
 
@@ -98,6 +99,7 @@ public void setApplicationContext(ApplicationContext applicationContext) throws
 	 * @see #getListenerContainerIds()
 	 */
 	@Override
+	@Nullable
 	public MessageListenerContainer getListenerContainer(String id) {
 		Assert.hasText(id, "Container identifier must not be empty");
 		return this.listenerContainers.get(id);

spring-kafka/src/main/java/org/springframework/kafka/listener/ListenerContainerRegistry.java

@@ -19,6 +19,8 @@
 import java.util.Collection;
 import java.util.Set;
 
+import org.springframework.lang.Nullable;
+
 /**
  * A registry for listener containers.
  *
@@ -36,6 +38,7 @@ public interface ListenerContainerRegistry {
 	 * @see org.springframework.kafka.config.KafkaListenerEndpoint#getId()
 	 * @see #getListenerContainerIds()
 	 */
+	@Nullable
 	MessageListenerContainer getListenerContainer(String id);
 
 	/**