Eliminating null fields in JSON output

The resolver pipeline seems to like explicit null fields. For example, if you do something like `query { things { id, name } }`, you might end up with something like:

```json
{
"results": {
"things": [
{"id": "1", "name": null},
{"id": "2", "name": null},
{"id": "3", "name": null},
{"id": "4", "name": null},
{"id": "5", "name": "a"}
]
}
}
```

but a frontend would generally be perfectly happy with:

```json
{
"results": {
"things": [
{"id": "1"},
{"id": "2"},
{"id": "3"},
{"id": "4"},
{"id": "5", "name": "a"}
]
}
}
```

Considering the `things` result only, that's a space saving of 20%. For large result sets, the aggregate size of all null values can be considerable. A real-world result set of ours is 273KB with nulls, but 189KB without nulls, which is a space saving factor of 41%.

I could do a pass over our results to eliminate nulls (possibly expensive, reflection-heavy and error-prone) or use a JSON encoder that eliminates nulls or allows filtering (not sure if that exists). But it would be a lot simpler if the resolver code could have a mode where object keys that are null wouldn't be emitted in the first place. Thoughts?