remove unneeded check for nil packets in session.sendConnectionClose
remove unneeded check for nil packets in session.sendConnectionClose

This was needed when logic in the packet packer was a lot more
complicated. Now that packing a CONNECTION_CLOSE is a separate code
path, this check isn't needed anymore.

konnykonny50/quic-go,lucas-clemente/quic-go,konnykonny50/quic-go,lucas-clemente/quic-go