JSON-LD 1.1 issue with dropping @language-only objects

json-ld-drop-langauge-only-issue.md

Test t0008:

{
  "@context": {
    "ex": "http://example.org/vocab#"
  },
  "@id": "http://example.org/test",
  "ex:test": { "@value": "test",  "@language": "en" },
  "ex:drop-lang-only": { "@language": "en" },
  "ex:keep-full-value": { "@value": "only value" }
}

expands to:

[
  {
    "@id": "http://example.org/test",
    "http://example.org/vocab#test": [ { "@value": "test", "@language": "en" } ],
    "http://example.org/vocab#keep-full-value": [ {"@value": "only value"} ]
  }
]

Based on the expansion algorithm steps 17 and 20, I'm not sure why ex:drop-lang-only is dropped.

Expansion 13.9 will recursively call expansion on "ex:drop-lang-only": { "@language": "en" }, and steps 17 and 20 will result in an empty array being returned.

I think 13.10 is where dropping is expect to happen, but that cannot occur because expanded value is not null, it's an empty array.

Step 17 needs to change to return null early instead of allowing step 20 to change it to an empty list.

w3c/json-ld-api#284