fix(plugin-seo): image generation handler invalid response

[r1tsuu]

Description

Fixed invalid raw (non json) response in generate-image endpoint caused to not generating the meta image.

Honestly tried to add e2e test because we don't have it for image-gen, but it doesn't pass even without my changes neither on Windows PC or Macbook, saying: ERROR There was a problem while uploading the file..

• [x] I have read and understand the CONTRIBUTING.md document in this repository.

Type of change

• [x] Bug fix (non-breaking change which fixes an issue)

Checklist:

• [x] Existing test suite passes locally with my changes

[github-actions[bot]]

This PR is stale due to lack of activity.

To keep the PR open, please indicate that it is still relevant in a comment below.