HTTP/3: skip empty request body buffers (ticket #2374).

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.