Writing mutations with null values in relations

Describe the bug

If you tries to execute a mutation with a null value in a relation, the mutation fails. It could be regression over https://github.com/nuwave/lighthouse/pull/1021.

Expected behavior/Solution

The mutation should work without working with the relation

Steps to reproduce

This mutation variables will work:

{
  "book": { 
    "upsert": [
      {
       "title": "test",
        "chapter": {
         "upsert": [{ "something": "value"}]
       }
      }
     ]
  }
}

This mutation variables won't work:

{
  "book": { 
    "upsert": [
      {
       "title": "test",
        "chapter": null
      }
     ]
  }
}

This mutation variables will work:

{
  "book": { 
    "upsert": [
      {
       "title": "test",
      }
     ]
  }
}

Output/Logs

Lighthouse Version

4.14.1

Reproduce the failing mutation with chapter: null, then start at src/Execution/Arguments/ResolveNested.php line 33 and src/Execution/Arguments/ArgPartitioner.php line 26, as named in the error. Trace how null nested arguments are partitioned, and verify that the mutation succeeds while leaving the relation untouched.