Should throw when passed undefined or null

[bakkot]

This was a normative change included as part of this commit, I'm guessing by accident. (That commit does what the Map constructor does, but we explicitly want to do something different.) I think the fix is to change the If _iterable_ is *undefined* or *null*, return _obj_ step to If _iterable_ is *undefined* or *null*, throw a TypeError exception and move it to the first step.

See discussion here.