DAL/Node: Fix advertisement of public port

What

Fix an issue where the port is not propagated correctly when --public-addr is used with the DAL node.

Why

It's a bug.

How

The advertising port is set by default to 11732. It should be overriden by the public port. However, @baugr believes (and I think his right) that the default should be set to None. This will be for another MR.

Manually testing the MR

We have tested via the following branch:

git checkout 4c7037fa
dune exec tezt/tests/cloud/main.exe -- cloud dal --ot -v --log-file /tmp/log --localhost --producers 1 --no-teztale --stake 1

Then going on:

http://localhost:16686

you should observe the producer receives a prune message from the bootstrap node (handle_prune trace). Without the patch, it does not since the connection is not made. The bootstrap node thinks he is already connected to the producer (while actually it is connected to the attester).