Mode for treating empty string the same as unspecified

[nreid260]

This is similar in spirit to https://github.com/remkop/picocli/issues/1987

Basically, I want to be able to write CLIs such that explicitly passing empty-string as a param value is the same as having not specified the parameter at all. (e.g. --bar=x --foo= is the same as --bar=x). I suggest empty string because it's the closest thing BASH has to as "null" value. I see this as the best practice for optional parameters in any language.

In particular, I want support for default values to work correctly. For example:

@Option(name = "--my_enum") MyEnum myEnum = MyEnum.A

tool --my_enum=

would invoke tool with MyEnum.A as the value for myEnum

My objective is to make it easier to wrappers/scripts/etc to invoke tools with default values, without having to know what the default values are. Wrappers should be able to assume empty-string is the default value for every param.

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Obviously this is a backward incompatible change, but could it be added as a setter on CommandLine?