> How do you avoid treating such candidates unfairly?

To be frank, the job of a hiring decision-maker isn't to treat candidates fairly, it's to hire the best fit for the role based on the available signals. It's the job of the candidate to send the best signals they can, however they go about doing that. One thing we'll probe for in our interviews with more experienced candidates is what kind of experience/lessons they have from their former employers, for example, and that can weigh in their favor if there's conceivable overlap with what we're doing.

But I would say don't give any single signal undue weight. Side projects are nice, but not mandatory. Heck, we've even hired people who bombed our programming test because they did okay in the post-test interview (where we'll usually see if they can work out where their solution failed).