How to query customField in GraphQL?

Question

I'm using a GraphQL API that accepts input with the following fields:

UpdateClientInput { 
  customFields [CustomFieldInput!]
  firstName String
  lastName String
  id ID!
}

The payload returns the following fields I'm interested in:

UpdateClientPayload {
  cardsOnFile [CardsOnFile!]
  firstName String
  lastName String
  id ID!
  ... (many, many more fields)
}

I'm trying to pass in an empty array to reset these cards on file. It seems like since the cards are not a part of the accepted input that might be impossible. I'm also concerned that customField is some specific GraphQL code constrained only to the API I'm using.

My questions:

1. Can you pass in an empty array in GraphQL like cardsOnFile: []?
2. Is it possible to pass in an input for one of these fields using the customField attribute? If so, how?

API I'm using for reference: https://developers.joinblvd.com/graphql-admin-api/api-reference/mutations/updateClient