[Backport 2.2] Fix minor issues in ui export converter classes

[dmytro-ch]

Original PR: #15692

Fix minor issues in \Magento\Ui\Model\Export\ConvertToXml and \Magento\Ui\Model\Export\ConvertToCsv classes.

Description

1. Fix the issue with the dynamically defined `$filter`` property.
2. Add missing throws to PHPDocs for methods.

Fixed Issues (if relevant)

N/A

Manual testing scenarios

N/A

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 @dmytro-ch. Thank you for your contribution.
Changes from your Pull Request will be available with the upcoming 2.2.6 release.

Please, consider to port this solution to 2.3 release line.
You may use Porting tool to port commits automatically.