Insufficied validation for `volumeMounts`

Using `kubeval --strict` still passes malformed StatefulSet (`volumeMount` item containing `emptyDir` key).

Helm install will fail with the following error:
```
ValidationError(StatefulSet.spec.template.spec.containers[1].volumeMounts[5]): missing required field "mountPath" in io.k8s.api.core.v1.VolumeMount]
```

Simplified example:
```
apiVersion: apps/v1
kind: StatefulSet
spec:
template:
spec:
containers:
volumeMounts:
- name: socket
emptyDir: {}
- name: log-buffer
emptyDir: {}
```