fix: feed image when using chained actions

[Soare-Robert-Daniel]

Summary

In #944 we allowed the custom tags to work with featured images. That condition established a default image for the chained action tags.

I fixed and re-write the condition to make its purpose more clear. Also corrected the serialization for image chained actions.

Note

The users affected by this issue were those using empty chained actions for the featured image.

Will affect visual aspect of the product

NO

Screenshots

Test instructions

• Create a new feed and add via Insert Tag the tag Item Image
• Also install the Pro version and check the image generation with OpenAI via chained actions.

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 #945

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