This question already has an answer here:
I am getting stuck already in few interviews when the companies ask me to show them my source code.
During 10 years of career all "my" source code is private, sealed by many NDAs source code that is nearly impossible I can show them.
Because I was so devoted at my professional work, I never had time to devote to any open source project. So no I did not contribute to any source project.
Is there any immediate solution? i.e. I know I could start creating an open source project, or contribute to an existing one, etc. But I see the effort of doing this as grounding a new company. So it is not happening immediately.