[admin-ui] Default variant not created when creating product

[zehawki]

Describe the bug
When a new product is created, it cannot be used directly since it does not appear in the variantList or variants key of the API response. Only variants carry price and stock and other info for add-to-cart and checkout etc.

To Reproduce
Steps to reproduce the behavior:

1. Go to Admin UI
2. Add a new product without variant
3. Exercise relevant API (eg ProductList)

        {
          "id": "55",
          "name": "New product A without variant",
          "slug": "new-product-a-without-variant",
          "description": "<p>This is a test for a new product being created in the Admin UI without variant being explicitly created. What will show in GraphQL?</p>",
          "featuredAsset": null,
          "variantList": {
            "totalItems": 0
          },
          "variants": []
        },

Expected behavior
When a new product is created, it would be good to have at least 1 variant created by default.

Environment (please complete the following information):

• @vendure/core version: v2

Additional context

1. This seems to be an inadvertent change in v2. In the admin-ui in V1 the variant is prefilled when creating a product.
2. https://discord.com/channels/1100672177260478564/1133746905860542474