Upgrade NullAway to 0.12.4

This commit also slightly refines nullness but without
significant user-side impact expected.

Closes gh-34525

spring-messaging/src/main/java/org/springframework/messaging/rsocket/service/RSocketServiceMethod.java

@@ -21,6 +21,7 @@ import java.time.Duration;
 import java.util.Iterator;
 import java.util.List;
 import java.util.Map;
+import java.util.Objects;
 import java.util.Optional;
 import java.util.function.Function;
 
@@ -166,7 +167,7 @@ final class RSocketServiceMethod {
 						((Mono<?>) responsePublisher).blockOptional());
 			}
 			else {
-				return (blockTimeout != null ?
+				return Objects.requireNonNull(blockTimeout != null ?
 						((Mono<?>) responsePublisher).block(blockTimeout) :
 						((Mono<?>) responsePublisher).block());
 			}