Terms cannot be deleted, once set

1. Goto <your-instance>/admin/application_settings/general#js-terms-settings
2. Insert some terms
3. Enable the terms
4. Disable the terms again
5. Try to delete the text in the TextBox
6. Hit save changes
7. Notice that the text is still there

Same happens with the GitLab application settings API

Proposal

• Allow deletion of the terms if un-enabled.
• Require presences of the terms if enabled. If a user submits empty terms when enabled, the validation error message should be: "You must define the terms."