chore(charts): fix object to list

Leaving default value ```{}``` makes error when trying to install chart. need to be a list to work properly with certificate names list.

deploy/charts/emqx/values.yaml

@@ -211,7 +211,7 @@ ssl:
   enabled: false
   useExisting: false
   existingName: emqx-tls
-  dnsnames: {}
+  dnsnames: []
   issuer:
     name: letsencrypt-dns
     kind: ClusterIssuer