When the Image Placeholder generator is set, new placeholders won't be generated, when a request comes from the back office.

Steps to reproduce
1. Clean installation of eZ Platform 2.5 or Ibexa DXP v3.2+
2. Set an image placeholder generator:

ezplatform:
    image_placeholder:
        default:
            provider: remote
            options:
                url_pattern: 'https://placekitten.com/%%width%%/%%height%%'

3. Login into the back office and create a new Image called Test.
4. Delete all files from <public_dir>/var/site/storage/images directory.
5. Refresh the Test Image view in the back office.

Result
Image Placeholder won't be generated.

Expected Result
Image Placeholder is generated normally.

Note
Image Placeholder is generated without issue when Test Image is accessed from the front office.