Degrade to default behavior if excerpt blank submitted

I think for some users, myself included, this is a very useful feature, but not something needed on every single node.

As such, it would be useful if the module reverted to the default Drupal behavior (truncated preview) if the Excerpt field provided by the module is left blank when nodes are created.

As it stands, if a node has a blank Excerpt with the module on and the node is promoted, the preview displays properly after the node is created, but on the front page the node appears blank.