Remove n+1 lookup values on stock item exist SQL call

Fixes:

```sql
Spree::StockItem Exists (4.7ms)  SELECT  1 AS one FROM "spree_stock_items"  WHERE 
"spree_stock_items"."deleted_at" IS NULL AND "spree_stock_items"."stock_location_id" = $1 AND 
"spree_stock_items"."variant_id" IN (1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 
1, 1, 1, 1, 1, 1, 1, 1, 1, .... 
```