Allow negative timestamps in publication_date_pubdate_validate()

Drupal core allows saving node creation times before 1970 (see #271398: Allow negative timestamps in node.created) but this module doesn't.

It does however use the value 0 as a default to indicate that the published date hasn't been set. If we allow any date and not just those past the epoch we'd have to use NULL for this purpose, and change the schema accordingly.