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
98b6491f0c