Comment 26 for bug 1532130

Looks like this has been fixed. Previously, doing what @reedobrien tried would cause the version string to disappear from the output of config-get.

(FWIW, all languages I'm aware of that have a null use it as a value. eg. the default value for uninitialized variables. Normally the default for 'unset' config items should be null, but we use the empty string to work around limitations in the juju command line)