Fix default for PVC selectors in GerritCluster

The default for selectors for PVCs configured by the GerritCluster
resource was documented with `{}`. While this is valid yaml, some
provisioners don't support selectors, e.g. the nfs-subdir-provisioner.
However, the default let to an inclusion of the selector term in the
PVC manifest, which led to failing PVC creation.

Giving `null` as the default resolves that issue.

Change-Id: Ida526a2c3177c723c58f6c49ecd9b038c63a8977
diff --git a/operator/README.md b/operator/README.md
index d3500df..b713568 100644
--- a/operator/README.md
+++ b/operator/README.md
@@ -171,7 +171,7 @@
 
     ## Selector (https://kubernetes.io/docs/concepts/storage/persistent-volumes/#selector)
     ## to select a specific persistent volume (optional)
-    selector: {}
+    selector: null
       # matchLabels:
       #   volume-type: ssd
       #   aws-availability-zone: us-east-1
@@ -186,7 +186,7 @@
 
     ## Selector (https://kubernetes.io/docs/concepts/storage/persistent-volumes/#selector)
     ## to select a specific persistent volume (optional)
-    selector: {}
+    selector: null
       # matchLabels:
       #   volume-type: ssd
       #   aws-availability-zone: us-east-1