Crystal started out as being more or less "compiled Ruby" (it's changed a bit since I first came across it). As such that means it uses "nil" just like Ruby doe

Crystal started out as being more or less "compiled Ruby" (it's changed a bit since I first came across it). As such that means it uses "nil" just like Ruby does. And Ruby, being heavily inspired by Smalltalk, took "nil" from Smalltalk. You'd probably have to ask Alan Kay to get to the bottom of this.