Allow unpublishing a page with blank publish_from

If page has publish_until set but publish_from is empty, user should
be able to unpublish it even before set date. If page isn't supposed
to be unpublished before said date, this is still easy to achieve by
filling in a value to publish_from field.
Merge pull request #4 from teppokoivula/master

Allow unpublishing a page with blank publish_from