'default_value' plugin does not run if 'get' returns null or empty value
The catch here is that you should not use the get process plugin explicitly. The behaviour you want is already implicitly used the default_value plugin. This is because get is the default process plugin and can be shorthanded. In your case this should work:

 field_privacy:
   plugin: default_value
   source: field_office_privacy
   default_value: 'open’


Also see documentation for get and default_value