Nullability refinements and related polishing (backported)

spring-messaging/src/main/java/org/springframework/messaging/handler/annotation/support/DefaultMessageHandlerMethodFactory.java

@@ -120,8 +120,7 @@ public class DefaultMessageHandlerMethodFactory
 	 * the ones configured by default. This is an advanced option. For most use cases
 	 * it should be sufficient to use {@link #setCustomArgumentResolvers(java.util.List)}.
 	 */
-	@SuppressWarnings("ConstantConditions")
-	public void setArgumentResolvers(List<HandlerMethodArgumentResolver> argumentResolvers) {
+	public void setArgumentResolvers(@Nullable List<HandlerMethodArgumentResolver> argumentResolvers) {
 		if (argumentResolvers == null) {
 			this.argumentResolvers.clear();
 			return;

spring-messaging/src/main/java/org/springframework/messaging/rsocket/annotation/support/RSocketFrameTypeMessageCondition.java

@@ -131,7 +131,6 @@ public class RSocketFrameTypeMessageCondition extends AbstractMessageCondition<R
 	 * @param message the current message
 	 * @return the frame type or {@code null} if not found
 	 */
-	@SuppressWarnings("ConstantConditions")
 	@Nullable
 	public static FrameType getFrameType(Message<?> message) {
 		return (FrameType) message.getHeaders().get(RSocketFrameTypeMessageCondition.FRAME_TYPE_HEADER);