Explicit literal NullValue on an input object gets wiped in compiled query

With schema:
```graphql
type Mutation {
edit(input: EditInput!): Boolean
}

input EditInput {
id: ID
prop: Float
}
```

This query:
```graphql
mutation tstEditMutation(
$id: ID!
) {
edit(input: {
id: $id,
prop: null
})
}
```

Is printed as following in a compiled file (notice missing `prop` field):
```graphql
mutation tstEditMutation(
$id: ID!
) {
edit(input: {id: $id})
}
```

Curiously, this query retains `prop`:
```graphql
mutation tstEditMutation {
edit(input: {
id: "123",
prop: null
})
}
```