One semi-workaround is to add 0 for phone number, though that won't deal with contacts not showing prior to that i.e. when no number was entered at all.
What will work though is look for an app called "contacts" or "people". That app will list all contacts including ones where no phone number was entered. That's the solution. So those contacts aren't lost.
If you don't see the "contacts" or "people" app, then you could check if it's installed under settings..apps..all. And you could try putting it in safe mode by holding the power button until it says "power off", and then when it says "power off", hold power off. Then it asks to go into safe mode, then say ok, and let it restart into safe mode. Then see if you see the app, to make sure a third party app wasn't blocking it.. hopefully you will see it. I saw it I think even in normal mode, it was just called "people" rather than "contacts. Whether it's "people" or "contacts" might depend on OS version.