Mailing header requires text but not html body

It's not required to include a text version of a CiviMail, and that's likely becoming rarer in comparison to pervasive use of html. Unfortunately, adding a header requires body text but not body html (wp-admin/admin.php?page=CiviCRM&q=civicrm%2Fadmin%2Fcomponent&action=add&reset=1).

I'd recommend making the body text optional, and perhaps adding a validation rule that (either body text or body html) is required.

This is a nice task for a new person on a sprint.