Updated CalculateExchangeLineItemInput and CalculatedExchangeLineItem Variant nullability

As of Admin GraphQL API 2025-01 version, CalculateExchangeLineItemInput.variantId and CalculatedExchangeLineItem.variant fields are nullable at schema level.

This is in preparation of future feature changes. In the current state CalculateExchangeLineItemInput.variantId will still be validated via a returned error if not present.

Learn more about CalculateExchangeLineItemInput and CalculatedExchangeLineItem.