editorial: correct AbstractModuleSource host invariants

[guybedford]

This resolves a question recently brought up by @dminor in ensuring the following corrections to the stated host properties on module sources:

1. Both prototype links are now described as "initial" state
2. %AbstractModuleSource% replaced with %AbstractModuleSource%.prototype to correct the invariant since it was clearly a bug to point to the constructor function itself
3. Instead of stating "when set", state "when not empty"

//cc @nicolo-ribaudo @legendecas

[nicolo-ribaudo]

I would also be interested in unifying the two definitions of source, updating this table to just say "a module source object", and defining a module source object as an object whose initial [[Prototype]] is an object whose initial [[Prototype]] is %AbstractModuleSource%.prototype.

[guybedford]

@nicolo-ribaudo I've updated the module source object definition to match this now.