GraphQL: Only fetch objects with a certain property equals `null`

I have a GraphQL query

query testQuery {
  books {
    title
    isbn
    publisher
    metadata {
      propertyA
      propertyB
      propertyC
    }
  }
}

In the GraphQL schema, metadata is an optional field. The other three are required, and all of them are string.

My question: I only want to filter items where the metadata is null or empty. I use GraphiQL.

For a string I can use where: {someString: {_isNull: true}}, but for a nested object this is not possible.