Fix TypeError in Shipping::collectRates() when no carriers are configured

[lbajsarowicz]

Description

Magento\Shipping\Model\Shipping::collectRates() iterates the carrier configuration without checking it is an array:

$carriers = $this->_scopeConfig->getValue(
    'carriers',
    \Magento\Store\Model\ScopeInterface::SCOPE_STORE,
    $storeId
);

foreach ($carriers as $carrierCode => $carrierConfig) {

Magento\Config\App\Config\Type\System::getDataByPathParts() returns null, not an empty array, when the requested key is absent from the merged scope data. That happens whenever no carrier module contributes a carriers/* node for the scope — for example with every shipping carrier disabled or uninstalled. On PHP 8 the foreach then throws a TypeError and checkout breaks. ScopeConfigInterface::getValue() is declared @return mixed, so callers cannot assume an array.

Note the asymmetry a few lines below in the same method: the $limitCarrier branch already normalises its value with is_array() before iterating. This applies the same guard to the branch that lacks it.

This is the same bug that was half-fixed in 2021

#30830 reported exactly this failure ("disable all shipping carrier modules" → blank checkout, array_keys(): argument #1 must be of type array, null given). It was fixed by #30822, which guarded the identical getValue('carriers', ...) call sites in Magento\Shipping\Model\Config::getActiveCarriers() and getAllCarriers() — but did not touch Shipping::collectRates(), which has the same unguarded pattern and is still reachable today.

Fixed Issues

No open issue; this is the remaining half of the defect reported in #30830, which was closed when #30822 fixed the sibling call sites.

Manual testing scenarios

1. Disable every shipping carrier (Stores > Configuration > Sales > Delivery Methods, or uninstall the carrier modules).
2. Add a product to the cart and open checkout, or call Shipping::collectRates() with a request that has no limitCarrier.
3. Before: TypeError: foreach() argument must be of type array|object, null given. After: rate collection completes and returns no rates.

Questions or comments

Gates run locally on 2.4-develop (Warden, PHP 8.3):

• Unit: Shipping/Test/Unit/Model/ShippingTest.php — 3 tests pass, including a new case that mocks the config to return null and asserts collectRates() returns normally instead of throwing.
• PHPCS Magento2: clean. PHPStan level 1: no errors.
• Negative check: the new test errors against unpatched Shipping.php and passes with the fix.

No signature changes, so no Semantic Version Checker exposure.

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds are green)

[lbajsarowicz]

@magento run all tests

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[lbajsarowicz]

@magento run Unit Tests