Membership price fields doesn't support non-required membership signups

When you're creating a contribution page with a membership signup component, there's a checkbox on the "Membership" tab labelled:
Require Membership Signup
If checked, user must signup for one of the displayed membership options before continuing.

However, if you use a price set for membership, this box is hidden, and CRM_Contribute_Form_Contribution_Main assumes that a price set means that a membership is ALWAYS required.

I don't see any reason why membership price sets shouldn't be optional, and in fact I have a client with this use case. It's a very simple conditional check, so I have a PR incoming to resolve this.