Should --arg= actually delete arg, not set it to empty?

[xparq]

The use case is repeating arg with --arg to override any prev. settings.

• Done in Empty (predicate) override option doesn't clear previous values #57

--arg can kinda still set to empy, albeit it's semantics is a predicate (bool), not "set to empty"!... It's "emptiness" just means [] returns true, but () still returns ""...

But... Even though the override logic should be fixed here (#16!), it's still up to the app, actually, to decide what to do with (arg) == "" or [app] == false, or any other values!

• Clean up the semantics!,,, It could be equally unexpected to some/in some cases to set --thing= but have [thing] as false!