Fix PHP 8.5 "null as array offset" deprecation in AbstractPdf::_getRenderer()

[Bashev]

Description

On PHP 8.5, using null as an array offset is deprecated ("Using null as an array offset is deprecated, use an empty string instead").

Magento\Sales\Model\Order\Pdf\AbstractPdf::_getRenderer() calls isset($this->_renderers[$type]) without guarding $type against null. When the renderer type resolves to null — e.g. a grouped-product order item whose getRealProductType() returns null — the isset() triggers the deprecation. In production mode, where deprecations are promoted to exceptions, PDF generation aborts (HTTP 404 / error page) instead of falling back to the default renderer.

This mirrors the other PHP 8.5 null-array-offset fixes already merged (e.g. #40889 for ScopeCodeResolver).

Fixed Issues

1. Fixes [PHP 8.5] AbstractPdf::_getRenderer() triggers "Using null as an array offset is deprecated" (breaks PDF print) #41134

Manual testing scenarios

1. Run Magento on PHP 8.5 in production mode (bin/magento deploy:mode:set production).
2. Place an order containing an item whose renderer type resolves to null (e.g. an order item where getRealProductType() returns null, such as some grouped-product scenarios or data migrated from older versions).
3. In the admin, open the invoice (or shipment/credit memo) for that order and click Print.
4. Before the fix: PDF generation aborts with an error page (the deprecation is promoted to an exception). After the fix: the PDF is generated normally, the item is rendered with the default renderer.

Covered by a new unit test AbstractTest::testGetRendererFallsBackToDefaultForNullType(), which promotes E_DEPRECATED to an exception so a regression fails the test:

vendor/bin/phpunit -c dev/tests/unit/phpunit.xml.dist app/code/Magento/Sales/Test/Unit/Model/Order/Pdf/AbstractTest.php

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)
• README.md files for modified modules are updated and included in the pull request if any README.md predefined sections require an update
• All automated tests passed successfully (all builds are green)

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[hostep]

Also can you edit your opening post and put a link to #41134 in there, so this PR and the issue are linked together? Thanks!

[Bashev]

@magento run all tests

[Bashev]

@magento run Database Compare, Functional Tests B2B, Functional Tests CE, Functional Tests EE

[Bashev]

@magento run all tests

[Bashev]

@magento run Functional Tests CE