If charm config field is undefined it should default to an empty string instead of undefined
undefined config values are no longer sent to juju on deploy
Fixed tests to check for sending undefined values on deploy
Merge pull request #534 from hatched/inspector-settings-1365205

Don't send 'undefined' when settings values are not defined.

Fixes #1365205
When deploying a service with undefined configuration values we were sending the string value "undefined" instead of nothing at all.