If a mailing list setting is missing, just don't send the mail.

Instead of being really insistent that all mailing list values are configured, just treat unset values as "don't send this kind of email".  In any case, the user will see a warning that no emails were sent, so an accidental misconfiguration shouldn't go unnoticed for long.

This change was suggested by @moy [here](https://github.com/mhagger/git-multimail/issues/39#issuecomment-27630867).