DOC: user manual should show how to configure null defaultValue and fallbackValue

[remkop]

Request from the picocli google group mailing list:

If a list option is specified on the commandline without a value I end up with an ArrayList with index 0 set to empty String.

For example for below cli, I want myList to be set an empty List or even just set to null.

java FallbackValueDemo -myList

@Option(
    names = "-myList",
    split = ",",
    arity = "0..1",
    fallbackValue = {}) // TBD
List<String> myList;

I can see how this could be useful.

This seems closely related to #879

If we go with the suggestion in #879, it becomes possible to specify {} as the fallback, which would mean an empty list. Seems doable.

[remkop]

The user manual documents that default value and fallback value can be set to null or Optional.empty() with Option.NULL_VALUE, but this information is not mentioned in the section on Default Values.

Add a subsection to Default Values to clarify how applications can get null or Optional.empty() default values or fallback values.