Resolve remaining nullability warnings

Issue: SPR-15869

spring-jms/src/main/java/org/springframework/jms/listener/adapter/MessageListenerAdapter.java

@@ -216,11 +216,6 @@ public class MessageListenerAdapter extends AbstractAdaptableMessageListener imp
 		// Regular case: find a handler method reflectively.
 		Object convertedMessage = extractMessage(message);
 		String methodName = getListenerMethodName(message, convertedMessage);
-		if (methodName == null) {
-			throw new javax.jms.IllegalStateException("No default listener method specified: " +
-					"Either specify a non-null value for the 'defaultListenerMethod' property or " +
-					"override the 'getListenerMethodName' method.");
-		}
 
 		// Invoke the handler method with appropriate arguments.
 		Object[] listenerArguments = buildListenerArguments(convertedMessage);