HTTP/3: skip empty request body buffers (ticket #2374). When client DATA frame header and its content come in different QUIC packets, it may happen that only the header is processed by the first ngx_http_v3_request_body_filter() call. In this case an empty request body buffer is added to r->request_body->bufs, which is later reused in a subsequent ngx_http_v3_request_body_filter() call without being removed from the body chain. As a result, rb->request_body->bufs ends up with two copies of the same buffer. The fix is to avoid adding empty request body buffers to r->request_body->bufs.