improve typings where '@id' can't be `undefined`

[elf-pavlik]

I can currently think of at least two scenarios:

1) object is created using fromSubject 2) shape requires IRI

The second case is exemplified in ShEx Primer As I understand it, given shape:

solidshapes:Event IRI {
  a [ schema:Event ] ;
  schema:name xsd:string
}

Every valid event would always have '@id'. I double-checked it with @ericprud, and he confirmed it.