The field is marked as optional:
https://github.com/kubernetes/kubernetes/blob/master/staging/src/k8s.io/api/core/v1/types.go#L2414

The comment also describes a use case for empty topologykey:
https://github.com/kubernetes/kubernetes/blame/master/staging/src/k8s.io/api/core/v1/types.go#L2410

But the validation rule says it's required:
https://github.com/kubernetes/kubernetes/blob/master/pkg/api/validation/validation.go#L2662