I have a table that belongs to either the Bill table or the Quotation table. Since each record requires either a Bill or a Quotation, the bill_id and quotation_id fields must be nullable. How can I define this in the Laravel Blueprint (https://github.com/laravel-shift/blueprint) draft.yaml file?

  CustomerInfo:
    name: string
    contact_tel: string
    email: string nullable
    address: string nullable
    postcode: string nullable
    city: string nullable
    state: string nullable
    relationships:
      belongsTo: Bill, Quotation