[Issue] Enforce array type in DesignTheme config processor

[m2-assistant]

This issue is automatically created based on existing pull request: #34092: Enforce array type in DesignTheme config processor

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Description (*)

Without the type check, an empty config node like <my_store /> in a config.xml breaks installation, as null is passed to the private method.

Related Pull Requests

none

Fixed Issues (if relevant)

none (can be created if desired)

Manual testing scenarios (*)

1. create module with store specific configuration in config.xml
2. use empty node for one store, like <my_store />
3. run bin/magento setup:install or integration test suite

Questions or comments

Since the changed method is private, this change is 100% backwards compatible. I'm not adding a test case since not having regressions should be a sufficient merge criteria when adding types.

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)