fix: list object must have user define

[adriantam]

Description

Need to specify list object user field must be non empty string. Currently, non-specified user field are interpreted as empty string. With the change, ListObject with payload

 {
  "type": "resource",
  "relation": "reader"
}

will return

{
    "code": "validation_error",
    "message": "invalid ListObjectsRequest.User: value length must be between 1 and 512 bytes, inclusive"
}

References

Close https://github.com/openfga/api/issues/58

Review Checklist

• [ ] I have clicked on "allow edits by maintainers".
• [ ] I have added documentation for new/changed functionality in this PR or in a PR to openfga.dev [Provide a link to any relevant PRs in the references section above]
• [ ] The correct base branch is being used, if not main
• [ ] I have added tests to validate that the change in functionality is working as expected

[jon-whit]

Needs buf format to be run on the sources