Fix null body handling in ResponseEntityResultHandler

This commit fixes `ResponseEntityResultHandler` so that it only tries to
call `writeBody` if the `ResponseEntity` is not null. In case the
response entity body is null, the response is flushed right away and the
request is signaled as handled.

Issue: SPR-14663

spring-web-reactive/src/main/java/org/springframework/web/reactive/result/method/annotation/ResponseEntityResultHandler.java

@@ -149,6 +149,10 @@ public class ResponseEntityResultHandler extends AbstractMessageWriterResultHand
 						.filter(entry -> !responseHeaders.containsKey(entry.getKey()))
 						.forEach(entry -> responseHeaders.put(entry.getKey(), entry.getValue()));
 			}
+			if(httpEntity.getBody() == null) {
+				exchange.getResponse().setComplete();
+				return Mono.empty();
+			}
 
 			String etag = entityHeaders.getETag();
 			Instant lastModified = Instant.ofEpochMilli(entityHeaders.getLastModified());

spring-web-reactive/src/test/java/org/springframework/web/reactive/result/method/annotation/ResponseEntityResultHandlerTests.java

@@ -65,6 +65,7 @@ import static org.junit.Assert.assertNull;
 import static org.junit.Assert.assertTrue;
 import static org.springframework.core.ResolvableType.forClassWithGenerics;
 import static org.springframework.http.ResponseEntity.ok;
+import static org.springframework.http.ResponseEntity.notFound;
 
 /**
  * Unit tests for {@link ResponseEntityResultHandler}. When adding a test also
@@ -168,6 +169,16 @@ public class ResponseEntityResultHandlerTests {
 		assertNull(this.response.getBody());
 	}
 
+	@Test
+	public void handleResponseEntityWithNullBody() throws Exception {
+		Object returnValue = Mono.just(notFound().build());
+		ResolvableType returnType = forClassWithGenerics(Mono.class, responseEntity(String.class));
+		HandlerResult result = handlerResult(returnValue, returnType);
+		this.resultHandler.handleResult(this.exchange, result).block(Duration.ofSeconds(5));
+		assertEquals(HttpStatus.NOT_FOUND, this.response.getStatusCode());
+		assertNull(this.response.getBody());
+	}
+
 	@Test
 	public void handleReturnTypes() throws Exception {
 		Object returnValue = ok("abc");