Better checking for default image for the RSS Block

[Soare-Robert-Daniel]

Summary

Added a checker that detects if the Feedzy default image is used when the thumbnail option is set to auto in the RSS block.

Will affect visual aspect of the product

YES

Screenshots

Test instructions

• Create a simple import with the URL: https://themeisle.com/blog/feed/
• The default feedzy image should be shown unless it is set to show.

Check before Pull Request is ready:

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)

Closes #920

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🎉 This PR is included in version 4.4.9 🎉

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