I didn't noticed that files were being handled by systemd-tmpfiles. When running the command above I discovered a problem with /var owner that made fail systemd-tmpfiles. Once I'd solved that the trigger file was created running 'dpkg-reconfigure nullmailer' and everything is running ok now.
I'm sorry as it was only a permission problem in my machine. In the first moment I'd think the creation of the trigger file was not handled at all, but it is, so we can close this bug now. Thank you for the help.
