@Override
    public HttpResponse convertResponse(ServiceRequestContext ctx,
                                        ResponseHeaders headers,
                                        @Nullable Object result,
                                        HttpHeaders trailers) throws Exception {
        if (result == null) {
            // Ignore the specified headers and return '204 No Content'.
            return HttpResponse.of(HttpStatus.NO_CONTENT);
        }

        return ResponseConverterFunction.fallthrough();
    }
A response converter implementation which creates an {@link HttpResponse} of {@link HttpStatus#NO_CONTENT}
when the specified {@code result} is {@code null}.
core/src/main/java/com/linecorp/armeria/server/annotation/NullToNoContentResponseConverterFunction.java