Focus on what you learned and how it made you a better developer, not on the project's star count or popularity.
The contribution itself matters less than what they learned: working with distributed teams, writing for an audience beyond their team, handling public code review, or solving problems outside their comfort zone. Candidates without open source experience can discuss personal projects or community involvement. No contribution is not a red flag if they show learning in other ways.
Reveals passion and self-motivation. Not having open source contributions is fine, but candidates should show evidence of learning beyond their day job in some form.