Unclear spec for array with fragments

In this page, there is an interactive example for playing with fragments.

availableProduce returns an array of Produce, which is an interface. Using fragments, we can match on only Fruit items.

Here is the query:

{
  mostPopularStall {
    availableProduce {
      ... on Fruit {
        name
        hasEdibleSeeds
      }
    }
  }
}

Then the response is:

{
  "data": {
    "mostPopularStall": {
      "availableProduce": [
        {
          "name": "pear",
          "hasEdibleSeeds": false
        },
        {
          "name": "blueberry",
          "hasEdibleSeeds": true
        },
        {
          "name": "banana",
          "hasEdibleSeeds": false
        },
        {},
        {}
      ]
    }
  }
}

Notice how the non-matching items (type Vegetable) have been returned as empty objects {}.

This behavior makes sense, however I was unable to find this documented in the spec.

What is the correct behavior here?

Sorry in advance if I have simply missed the relevant section!

graphql / graphql-spec

Unclear spec for array with fragments #1079