Send null values in config and params as `null`

Now that Juju supports null configuration values the GUI needs to stop converting `null` values to "null". As a side effect all params for websocket rpc calls also send nulls for values which don't exist. This doesn't appear to have any negative side effects.

This is a fix for: https://bugs.launchpad.net/charms/+source/postgresql/+bug/1390858