Fixes #1026

Changes code so that it treats `undefined`, `null` and `nil` differently when creates subclasses off it. Purposely created `nil` subclasses are outputted with `null` put as superclass in .js file. Thus, missing PascalCase globals while loading .js file are detected via `globals.PascalCase` being `undefined` and missing PascalCase globals while importing .st file are detected as being `nil` (ProtoObject is still outputted a warning when recompiling Kernel-Objects, a minor nuisance). When `null` is subclassed, no warning is outputted.