Replace empty strings with undefined in default values of configuration settings

[priyamsahoo]

The idea is to replace all the empty strings with undefined values in the extension settings. The setting should be undefined if the user is not assigning any value to it. Related: https://github.com/ansible/vscode-ansible/issues/661

[ssbarnea]

This might be a serious problem because 99% of the users will not know how to reset a particular configuration item to its default value, instead thew will only empty the settings value ... aka putting an empty string.

From the user point of view, empty value is the same as undefined.