title: how to get ruamel yaml emit blank instead of null?
body: <p>Using ruamel.yaml, the emitted yaml is created like so:</p>
  <pre><code>from ruamel.yaml import YAML
  
  ydata = dict(
    apiVersion='compliance.openshift.io/v1alpha1',
    disableRules=None,
    extends='tbd',
    kind='TailoredProfile',
    metadata=dict(name='Best Practices NIST CONTROLS Profile'),
    setValues=dict(v1='tbd'),
    title='tbd',
  )
  
  yaml = YAML(typ='safe')
  yaml.default_flow_style = False      
  with open(output_filepath, 'w') as outfile:
      yaml.dump(ydata, outfile)
  
  </code></pre>
  <p>and looks like this:</p>
  <pre><code>apiVersion: compliance.openshift.io/v1alpha1
  disableRules: null
  extends: tbd
  kind: TailoredProfile
  metadata:
    name: Best Practices NIST CONTROLS Profile
  setValues:
    v1: tbd
  title: tbd
  </code></pre>
  <p>But I want <strong>null</strong> to be blank, like so:</p>
  <pre><code>disableRules: 
  </code></pre>
  <p>How can that be done?</p>