I happened to find that empty Dcc headers are not removed by the command 'post' and that they are delivered intact.  This can be
easily confirmed by submitting an e-mail with an empty Dcc header 'Dcc: '.

This is caused by the following part of uip/post.c.
Since Dcc has the HNIL flag, an empty Dcc header is written out to the delivering message as it is.

937    if (count < 1) {
938        if (hdr->flags & HNIL)
939            fprintf (out, "%s: %s", name, str);
940        else {
941            /*
942             * Sender (or Resent-Sender) can have only one address
943             */

This could be fixed by checking whether the header is Dcc or Bcc as follows:

       if (count < 1) {
           if (hdr->flags & HNIL) {
               if ( !(hdr->flags & (HBCC | HDCC)) ) {
                   fprintf (out, "%s: %s", name, str);
               }
           }
           else {
               /*
                * Sender (or Resent-Sender) can have only one address
                */

The same behavior applies to empty Bcc headers too.
However, they are not delivered because the command 'sendmail' that is invoked by the command 'post' removes Bcc headers while Dcc headers are kept intact.

I hope this information is useful.

Last but not least, I would like to express my best gratitude to those contributing to this project since NMH is my main and best MUA.