[fixed] KC: dagent "lmtp_port", "", CONFIGSETTING_NONEMPTY breaks usage of new option lmtp_listen

Hello KC::devs,
commit 82381ef1ea5
declares dagent option lmtp_port legacy in favour of lmtp_listen.
So, we either set lmtp_port/ server_bind/ server_intf or lmtp_listen.

This collides with dagent.cpp, ln. 3407


as an unset lmtp_port-option will fail during dagent-start:

[crit   ] Config error: Option 'lmtp_port' cannot be empty!

A current workaround is to define lmtp_port = "".