empty applies_to tag strangeness

[shainaraskas]

Describe the bug

if I just put this in the front matter:

applies_to:

I get this:

IMO we rarely need this, because we only include stack labels alongside deployment type labels in limited circumstances, and usually with version info.

if we did want it, we wouldn't want the little empty blob representing a generic product label.

Expected behavior

CI error on empty applies_to tag

Steps to reproduce

front matter of

---
applies_to:
---

Tooling

• docs-builder
• migration tooling
• I'm not sure

[theletterf]

This should be addressed by #1245