Allow image resizing if `current_user=nil`

What does this MR do?

To run the experiment https://gitlab.com/gitlab-org/gitlab/-/issues/241533, we need to allow anonymous image resizing requests.

Image Resizing experiment sits under the feature flags described in !40606 (merged).

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• [-] Separation of EE specific content

Availability and Testing

Tested locally in incognito mode.

Security

N/A

Related to #241533