Form Templates - Display error when saving a template with an empty static signer type

Currently, if a form template has a static signer and you save the form no error is shown and it appears to save, but if you open the builder it will not have the signer listed as a choice. If you then go back to edit the template, that static signer will no longer be listed.

Instead of allowing an empty static signer to be saved, we should display an error to prevent confusion.