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

[vivekkumarcedcoss]

Original Pull Request

#20231

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-engcom-team]

Hi @vivekkumarcedcoss. Thank you for your contribution
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[rogyar]

I cannot reproduce this issue on 2.2-develop branch. Looks like the issue/fix is fair for v2.3 only. isDirectivesExists method was added to app/code/Magento/Catalog/Helper/Output.php only for v2.3.
Please, feel free to reopen this PR if you have a way to reproduce the issue on v2.2. Thank you.

[ghost]

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