Model architecture: Allow traits in strata to be null

[kring]

Any trait in a stratum should be allowed to have the value null. It means "even if a strata below me has provided a value for this trait, make the value undefined". It is also used to mark removal of things from lists. For example:

{
  "info": [
    {
      "name": "Service Description",
      "content": null
    }
  ]
}

means "remove the info section with the name Service Description".

This is currently done in an ad-hoc fashion with the isNullable property of @primitiveTrait, but it should be allowed everywhere.

Note that the flattened and Model views of traits should never have the value null. If a trait has no value, either because one was never given or because it was removed by a stratum, then that trait's value will be undefined.

[soyarsauce]

isRemoval also currently exists for this