fix(catalog): guard empty website_id in tier price validation (#40799)

[lbajsarowicz]

Description

When a website is unassigned from a product in admin, the tier_price form
may post rows with an empty website_id. AbstractGroupPrice::validate
then evaluates $rates[$priceRow['website_id']]['code'] which raises
Warning: Undefined array key "" and short-circuits the save —
Product is not saved, the validate endpoint returns error: true.

Skip rows whose website_id is missing or empty (already skipped when
=== 0 / global scope), and skip rows whose website_id has no entry in
the rates map.

Fixes #40799

Fixed Issues

• Fixes Warning: Undefined array key "" in vendor/magento/module-catalog/Model/Product/Attribute/Backend/GroupPrice/AbstractGroupPrice.php on line 248 when saving product #40799: Warning: Undefined array key "" in AbstractGroupPrice.php on line 248 when saving product

Manual testing scenarios

1. Set up multi-website store with advanced pricing on a product (tier prices per website/customer group).
2. Save product with tier prices across two websites.
3. Unassign one website from the product, save.
4. Reload product edit page and save again — saves successfully (before: validate endpoint returns the Undefined array key "" warning and product is not saved).

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All automated tests passed successfully (43 existing tests pass)
• Changes to the codebase comply with technical guidelines

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[lbajsarowicz]

@magento run all tests