Don't try and construct uploader email from system hostname

This fixes a bug where, if no EMAIL, DEBEMAIL or maintainer option is set, an invalid Uploader field is constructed from just the hostname. dak handled this not very gracefully, crashing due to https://salsa.debian.org/ftp-team/dak/-/blob/master/daklib/textutils.py#L113, with the end result that no email was sent back, even though the command was successful otherwise. ftp-master is aware of the issue.

Probably the intent in was to include username@ - but there's no actual requirement to include an Uploader field, or set email address for any other reason, so just drop the defaulting of email and correct the validation logic instead. I have successfully used this with EMAIL unset to send a commands file with no Uploader field, which was processed by dak and a return email correctly sent to the email address in the GPG keyring.