Remove runtime-config from kubeadm if empty (#6311)

roles/kubernetes/master/templates/kubeadm-config.v1beta2.yaml.j2

@@ -158,7 +158,7 @@ apiServer:
     encryption-provider-config: {{ kube_cert_dir }}/secrets_encryption.yaml
 {% endif %}
     storage-backend: {{ kube_apiserver_storage_backend }}
-{% if kube_api_runtime_config is defined %}
+{% if kube_api_runtime_config|length > 0 %}
     runtime-config: {{ kube_api_runtime_config | join(',') }}
 {% endif %}
     allow-privileged: "true"