chore: ignore empty condition in type restriction definitions

[jon-whit]

Description

The validation rule should only apply to the condition in a RelationReference (which is used for type restrictions) if the condition is not empty (e.g. ""). This ensures that is the case.

References

Review Checklist

• [x] I have clicked on "allow edits by maintainers".
• [x] 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]
• [x] The correct base branch is being used, if not main
• [x] I have added tests to validate that the change in functionality is working as expected

[jon-whit]

diff on the OpenAPI is alright - it's a false positive.