Shopify Discount Sheet — Hotfix 1.0.1

Fixed:
- "Shopify HTTP 400: Invalid variables parameter" when clicking
  Test Shopify Connection.
- Empty GraphQL variables are now sent as {} instead of [].

Existing installation:
1. Upload the included includes/shopify.php file.
2. Replace your current includes/shopify.php.
3. Do NOT replace config.php, so your shop domain, token and password stay unchanged.
4. Refresh the page and click Test Shopify Connection.
