[Woo POS] [Variable Products] Add empty state for variation lists

[joshheald]

Description

Add an empty state for variations, to be shown when a parent with no enabled variations is selected.

This should be similar to the Products empty state, but refer to variations and leave the cart visible.

Ref: woocommerce/woocommerce-android#13209

[dangermattic]

Thanks for reporting! 👍