Updating default from null to ""

roles/openshift_metrics/defaults/main.yaml

@@ -16,7 +16,7 @@ openshift_metrics_hawkular_nodeselector: ""
 openshift_metrics_cassandra_replicas: 1
 openshift_metrics_cassandra_storage_type: "{{ openshift_hosted_metrics_storage_kind | default('emptydir') }}"
 openshift_metrics_cassandra_pvc_size: "{{ openshift_hosted_metrics_storage_volume_size | default('10Gi') }}"
-openshift_metrics_cassandra_pv_selector: "{{ openshift_hosted_metrics_storage_labels | default(null) }}"
+openshift_metrics_cassandra_pv_selector: "{{ openshift_hosted_metrics_storage_labels | default('') }}"
 openshift_metrics_cassandra_limits_memory: 2G
 openshift_metrics_cassandra_limits_cpu: null
 openshift_metrics_cassandra_requests_memory: 1G