invalid value: \"null\": status.creationtimestamp in body must be of type string

[shenkonghui]

https://pkg.go.dev/sigs.k8s.io/cluster-api/api/v1alpha2#Convert_v1alpha2_ClusterStatus_To_v1alpha3_ClusterStatus

During the upgrade to controller-tools@v2 for v1alpha2, we noticed a failure would occur running Cluster API test suite against the new CRDs, specifically spec.metadata.creationTimestamp in body must be of type string: "null". The investigation showed that controller-tools@v2 behaves differently than its previous version when handling types from metav1(k8s.io/apimachinery/pkg/apis/meta/v1) package.

In more details, we found that embedded (non-top level) types that embedded metav1.ObjectMeta had validation properties, including for creationTimestamp (metav1.Time). The metav1.Time type specifies a custom json marshaller that, when IsZero() is true, returns null which breaks validation because the field isn't marked as nullable.