While contributing to open source projects would probably give you some advantage with some interviewers in some environments, not having them is unlikely to detract for the other interviewers or environments out there.
Some specific environments or interviewers may place more emphasis on the open source projects they use or contribute to, but if your CV is otherwise good, you are unlikely to suffer for not being involved with those projects.
As always though, you have to tailor your job application to the job you are applying for. Emphasise areas they are likely to be interested in, de-emphasise areas they are unlikely to be interested in.
Interviewing for my current position, I emphasised the work I once did on a Extreme ultraviolet microscope, as I knew that my prospective employers would know what that involves. I made it clear that while I wasn't an optical or UHV engineer, I had, as a software engineer, an appreciation of reflective (rather than refractive) optics and ultra-high vacuum systems. This almost certainly made me stand out from the crowd.
Finally, just because you can't show them the code (or sometimes even talk about the product in any detail), doesn't mean you can't talk about the the design decisions you had so make, and the effects you had on the product.
From my own experience I might explain that:
- In my first job after university I re-factored a military GIS client/server system so that it had constant performance over all zoom ranges rather than getting exponentially slower as the user zoomed out.
- I took on a project which was already 6 months late, where no software had been written and successfully delivered the most important functionality by the time the hardware was ready for live testing.
- I successfully ported a machine vision application from an obsolete library to modern one, adding Firewire and Gig-E Vision camera support while retaining backwards analogue camera compatibility.
and so on.