MimeParser.offer needs to ignore empty buffers

[romain-grecourt]

Environment Details

• Helidon Version: 2.6.0
• Helidon SE

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When processing a multipart server payload from a client response, Netty's LastHttpContent empty buffer is not expected and triggers a MimeParserException Invalid state: END_MESSAGE.

Reproducer:

MultiPartSupport multiPartSupport = MultiPartSupport.create();
WebClient webClient = WebClient.builder()
                               .baseUri("http://localhost:8080")
                               .addMediaSupport(multiPartSupport)
                               .build();

WebServer.builder()
         .port(8080)
         .addMediaSupport(multiPartSupport)
         .addRouting(Routing.builder()
                            .get("/test", (req, res) ->
                                    res.send(WriteableMultiPart.builder()
                                                               .bodyPart("foo", "bar")
                                                               .bodyPart("bob", "alice")
                                                               .build()))
                            .get("/", (req, res) ->
                                    webClient.get()
                                             .path("test")
                                             .accept(MediaType.MULTIPART_FORM_DATA)
                                             .submit()
                                             .flatMap(response -> response.content().asStream(ReadableBodyPart.class))
                                             .forEach(ReadableBodyPart::drain)
                                             .onError(req::next)
                                             .forSingle(res::send))
                            .build())
         .build()
         .start();