SKU field should be optional when create new product variant inside Manage variants

[Dominic-Preap]

Describe the bug

In product detail, when we try to add new option values and create new product variant, the SKU field is required. This should not be like that as when create new product the first time or when update product variants, the SKU can be optional.

To Reproduce
Steps to reproduce the behavior:

1. Go to Product detail
2. Go to Manage variants
3. Click Create product variant
4. Fill info except SKU but it's required

Expected behavior

SKU should be optional just like when create new product with Product variants.

Environment (please complete the following information):

• @vendure/core version: 3.0.0
• Nodejs 20
• Database: mysql

Additional context
N/A