Act more graceful when a new secret entry is created programatically

Just discovered this module. It's great. Hope it won't get too bloated one day with too many exotic features/field types.

One small cosmetic problem. If I create some entries (without their values) programatically or via drush in the config like so:

    parameters.collection.global:
        parameters:
            google_geocode_api_key:
                name: google_geocode_api_key
                label: Google Geocode API Key
                type: secret

The UI shows PHP warnings on initial load due to the missing "encoding:" part. If there is no value yet I think it should act more graceful. The warning vanishes as soon as the user enters a value for the first time. Nevertheless, I think this could be handled a bit better.