Whether or not you can contribute is an easy no, you need self employed income to contribute to a self employed retirement plan.
What rises to the level of a business to qualify for opening a Solo 401(k) is a trickier question. Some people want to do this to roll over a traditional IRA so they can make backdoor Roth contributions without running into the pro rata rule. I've seen people on bogleheads say they've done it based on selling something on ebay or craigslist, and other people caution them against it. Personally, if I would only feel comfortable opening a solo 401(k) in a year I filed a Schedule C for business income, but I'd probably feel comfortable keeping it open thereafter even if I didn't have business income every year as long as there was some chance that I would.
