SKUs when not required are still auto-populated when not required

[CWFranklin]

• Lunar version: 0.1.0
• Laravel Version: 9.32.0
• PHP Version: 8.1.10
• Database Driver & Version: mysql

Steps To Reproduce:

• Mark SKU as not required (but still unique) in Product config
• Create new product, add variants
• Create another new product, add the same variants again

Expected Behaviour:

Don't auto-populate SKUs when they are not required. When they are required, populate with useful values that aren't duplicated by this system.

Actual Behaviour:

SKUs are auto-populated with -1, -2 etc. These are auto-populated with the same values on each product and cause conflicts and therefore errors when saving the product itself or the variants.