[Backport] Fix app/code/Magento/Backend/Block/Media/Uploader.php getConfigJson() method

[mageprince]

Original Pull Request

#11665

This method is using an undefined class property _coreData, that method cannot work, only crash if called, since it will try to call jsonEncode() method from a null property (even if defined dynamically).

Description

Adapted to use \Magento\Framework\Json\EncoderInterface instead of (probably) legacy code, the way it's done at \Magento\Downloadable\Block\Adminhtml\Catalog\Product\Edit\Tab\Downloadable\Links::getConfigJson.

Fixed Issues (if relevant)

It may be related with some open issue, but after searching, I didn't find one that may be caused by this method.

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 @mageprince. Thank you for your contribution
Here is some useful tips how you can test your changes using Magento test environment.
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[magento-engcom-team]

Hi @mageprince. Thank you for your contribution.
We will aim to release these changes as part of 2.2.6.
Please check the release notes for final confirmation.

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