Conflict between @vocab and blank nodes

[joachimvh]

Minimal example (based on CommunitySolidServer/CommunitySolidServer#1643):

{
  "@context" : {
    "@vocab": "_:"
  },
  "@type": "OPENAIRE",
  "@id": "https://scholexplorer.openaire.eu/#about"
}

When parsing the above example, the parser will interpret the object of the @type triple as a NamedNode with value _:OPENAIRE, causing incorrect results when serializing this again. The JSON-LD playground interprets it as an actual blank node.