how to keep null value in yaml file while dumping though ruamel.yaml

I have YAML file site.yaml:

Kvm_BLOCK:
  ip_address: 10.X.X.X
  property: null
  server_type: zone

loaded and then dumped with:

ruamel.yaml.dump(site_yaml, new_file, Dumper=ruamel.yaml.RoundTripDumper)

it becomes

Kvm_BLOCK:
      ip_address: 10.X.X.X
      property: 
      server_type: zone

how to retain this null value in property block