compaction: 12.7.1 empty array to 7.3 IRI compaction candidate

During a refactor of my compaction implementation I ended up with an unfortunate interaction where instead of passing the empty array as the value through in 12.7.1, a null ended up going through. This then caused the wrong candidate to be selected in IRI compaction in 7.3, due to the value is null check.

The test suite lacks coverage for this, but a reproducer looks like this:

{
  "@id": "http://example.org/id",
  "http://example.org/prop": []
}

{
  "ex": "http://example.org/",
  "ex:prop": { "@id": "http://example.org/prop", "@container": "@list" }
}

{
  "@context": {
    "ex": "http://example.org/",
    "ex:prop": { "@id": "http://example.org/prop", "@container": "@list" }
  },
  "@id": "ex:id",
  "http://example.org/prop": []
}

If a nil/null is passed through instead of the empty array, you end up with ex:prop instead of http://example.org/prop.

This could potentially be folded into t0004?

Start with the supplied JSON-LD reproducer and the existing test suite, checking whether this case belongs in t0004. Trace the empty-array value through compaction steps 12.7.1 and 7.3, then add regression coverage showing that the full IRI candidate is selected instead of the compact term.