Unable to add a feature to a release pipeline via the API (500 error)

### How are you running Flagsmith

- [ ] Self Hosted with Docker
- [ ] Self Hosted with Kubernetes
- [x] SaaS at flagsmith.com
- [ ] Some other way (add details in description below)

### Describe the bug

i'm trying this endpoint {{baseUrl}}/projects/:project_id/release-pipelines/:release_pipeline_id/add-feature/ and getting a Server Error (500) in response. via the UI i can add a feature to a pipeline, but via the API i get this error without more info..

Sentry error:
:red_circle: [ValueError](https://flagsmith.sentry.io/issues/6946363834/events/840602872ef1457ab5724e6b696db745/?referrer=slack)
/api/v1/projects/{project_pk}/release-pipelines/{pk}/add-feature/
Cannot assign "": "AuditLog.author" must be a "FFAdminUser" instance.

### Steps To Reproduce

1. Create a pipeline
2. Create a feature
3. Click "add to pipeline" and select your pipeline
4. Works!
5. Create a new feature
6. Use the API endpoint to try to add it to the pipeline
7. Fails...

### Expected behavior

see above

### Screenshots

_No response_