Fixed behavior of 'assets' and 'images' fields in Product Variant Tailorings

We made the assets and images fields of entities related to Product Variant Tailoring optional to match the documented behavior.

Changes:

• [API] Made the images field in the Set Images update action optional.

• [GraphQL API] Made the images and oldImages fields in the ProductTailoringImagesSet type optional.

• [GraphQL API] Made the images input field in the ProductTailoringImagesSet type optional.

• [GraphQL API] Made the images and assets fields in the VariantTailoring and ProductVariantTailoring types optional.

The following changes were introduced in terms of GraphQL SDL:

type ProductTailoringImagesSet implements MessagePayload {
  variantId: Int!
  storeRef: KeyReference!
  productRef: Reference!
  productKey: String
  oldImages: [Image!]
  images: [Image!]
  type: String!
}

type ProductVariantTailoring {
  id: Int!
  images: [Image!]
  assets: [Asset!]
  attributesRaw: [RawProductAttribute!]!
}

input SetProductTailoringImages {
  variantId: Int
  sku: String
  images: [ImageInput!]
  staged: Boolean = true
}

type VariantTailoring {
  images: [Image!]
  assets: [Asset!]
}