Multi-head connections does not support nullability

[SudoBobo]

Consider the schema:

"hero": {
    "name": "hero",
    "type": "record",
    "fields": [
        { "name": "hero_id", "type": "string" },
        { "name": "hero_subtype_id", "type": "string*" },
        { "name": "hero_type", "type": "string*" },
    ]
},
"human": {
    "name": "human",
    "type": "record",
    "fields": [
        { "name": "human_id", "type": "string" }
    ]
},
"starship": {
    "name": "starship",
    "type": "record",
    "fields": [
        { "name": "starship_id", "type": "string" }
    ]
}

And collections:

"hero_collection": {
    "schema_name": "hero",
    "connections": [
        {
            "name": "hero_connection",
            "type": "1:1",
            "variants": [
                {
                    "determinant": {"hero_type": "human"},
                    "destination_collection": "human_collection",
                    "parts": [
                        {
                            "source_field": "hero_subtype_id",
                            "destination_field": "human_id"
                        }
                    ],
                    "index_name": "human_id_index"
                },
                {
                    "determinant": {"hero_type": "starship"},
                    "destination_collection": "starship_collection",
                    "parts": [
                        {
                            "source_field": "hero_subtype_id",
                            "destination_field": "starship_id"
                        }
                    ],
                    "index_name": "starship_id_index"
                }
            ]
        }
    ]
},
"human_collection": {
    "schema_name": "human",
    "connections": []
},
"starship_collection": {
    "schema_name": "starship",
    "connections": []
}

We can have a hero object like:

hero_collection:
- hero_id: hero_id_3

The above query is expected to work, but it does not:

{ hero_id = "hero_id_3" }

query obtainHeroes($hero_id: String) {
    hero_collection(hero_id: $hero_id) {
        hero_id
        hero_type
        hero_connection {
            ... on box_human_collection {
                human_collection {
                    human_id
                }
            }
            ... on box_starship_collection {
                starship_collection {
                    starship_id
                }
            }
        }
    }
}

Executing produces an error:

errors:
- message: 'Parent object of union object doesn''t have determinant fields which are
    necessary to determine which resolving variant should be used. Union parent object:

    "--- {''hero_type'': null, '''hero_subtype_id'': null,
    ''hero_id'': ''hero_id_3''}

    ...
    Determinant keys:

    "---

    - hero_type

    ...

[Totktonada]

I want to clarify a bit why I think hero_type should be allowed to have auxiliary value when hero_subtype_id is null. It is possible that we don't want to set any specific type for a hero when it has no connected part.

Now we determine 'orphan connection' inside a resolve function of a certain connection variant. But it is possible to determine it before choosing a variant in case when determinant fields are the same for all connection vairants.