Ghost/ghost/admin/app/transforms/json-string.js

import Transform from '@ember-data/serializer/transform';

export default Transform.extend({
    deserialize(serialized) {
        let _serialized = serialized === '' ? null : serialized;
        return JSON.parse(_serialized);
    },
    serialize(deserialized) {
        return deserialized ? JSON.stringify(deserialized) : null;
    }
});