The text box has validation that does not allow you to save with no content. You can get around this by adding a small HTML comment. The HTML comment will not show up in the email client, because it is rendering the email in HTML and the comments are omitted. Here's how to do it.
1. Go to Admin and click Email.
2. Select all of the text and press the Delete key.
Before deleting, copy/paste this into a text document as a backup.
3. Type in <!--This is a comment -->.
4. Click Save Changes.
For the Unsubscribe Text you have to add a single character. Use a dash or a period.