Fix #20229-Fatal error on product-compare page when attributes not configured

[vivekkumarcedcoss]

Description (*)

The productAttribute expects second parameter to a string which was failed when $block->getProductAttributeValue($item, $attribute) returns object due to empty value of attribute.
Added a check of is_string since it expect second param to a string.

Update list.phtml for issue #20229

Fixed Issues (if relevant)

1.Issue: #20229
2. ...

Manual testing scenarios (*)

1. ...
2. ...

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 on Travis CI are green)

[magento-cicd2]

All committers have signed the CLA.

[magento-engcom-team]

Hi @vivekkumarcedcoss. Thank you for your contribution
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[magento-engcom-team]

Hi @aleron75, thank you for the review.
ENGCOM-3946 has been created to process this Pull Request

[ghost]

Hi @vivekkumarcedcoss, thank you for your contribution!
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[magento-engcom-team]

Hi @vivekkumarcedcoss. Thank you for your contribution.
We will aim to release these changes as part of 2.3.1.
Please check the release notes for final confirmation.

[Brengineer]

Note that this solution changes existing functionality: For non-configured attributes, instead of echoing "No" or "N/A" as it was before, the output will now simply be blank. In order to retain current functionality, instead of checking for is_string, the \Magento\Framework\Phrase object should be cast as a string. (Although this is not necessarily a problem, I thought it should be mentioned.)