> A "portfolio" does not have to, and should not, consist of side projects.

I agree with this

> If you have absolutely nothing that you can share (code, screenshots, a URL, a "case study") with a prospective employer, it's not the prospective employer that has a problem.

I disagree with this. If you require this to be true, you've instantly excluded everyone's experience in either government (particularly defense), or industry when you work on internal services. There's a huge number of enterprise developers at my current company who will never be able to show you their work because they're all internal applications. Maybe a "case study" would work, but you have to be real careful of not to cover anything that doesn't fall into the trade secret category.