Headerless Panel has additional marginTop

[bobbyshaw]

trafficstars

Describe the bug

Panels without a header but just content now have a marginTop.

Due to this change

As per @leeBigCommerce's comment in the PR, I'm not sure about the full list of use cases you're prepared to support in the Panel component or whether your recommendation would be for us to rebuild with a Box (probably what we'll do in this instance but thought I'd share in case it affects others).