Send error signal also for empty server responses

Prior to that commit, the `ResponseSpec` `WebClient` would process error
responses (4xx, 5xx HTTP status) and transform those into error signals
with a `WebClientResponseException`. But this would only work if the
HTTP response would have a non-empty response body.

An empty error response would not send an error signal and only
translate in an `onComplete` signal.

This commit fixes this behavior and makes sure that this error signal is
sent in all cases.

Issue: SPR-15946

spring-webflux/src/main/java/org/springframework/web/reactive/function/client/DefaultWebClient.java

@@ -483,6 +483,7 @@ class DefaultWebClient implements WebClient {
 						DataBufferUtils.release(dataBuffer);
 						return bytes;
 					})
+					.defaultIfEmpty(new byte[0])
 					.map(bodyBytes -> {
 						String msg = String.format("ClientResponse has erroneous status code: %d %s", response.statusCode().value(),
 								response.statusCode().getReasonPhrase());