String option is empty string by default and not null

[georgmittendorfer]

In spring shell 3.x an option @Option(longName = "test", description = "some description") String test was null when not specified. In spring shell 4.x it is an empty string "". That breaks old behavior and seems to be counter intuitive for me. I assume it is a bug. If that is wanted behavior feel free to close this issue.

[piotrooo]

I think this is a more complex issue than just handling strings and is related to general type and value handling (ref: #1262).

We should rethink how we handle all possible option types and their values.