Enable option with empty argument

[plar]

This PR will allow for passing an empty argument to the option and will delegate the logic for managing the empty argument to the user side. See the full discussion here.

Now, we can call command.NewOptionWithArgument("--opt", ""), and it will generate --opt=, which is a legitimate case. The previous implementation skipped generating --opt if argument was an empty string ("").

This logic has now been transferred to the user side for handling empty arguments.

// OptWithArg("") => --opt=
// OptWithArg("custom") => --opt=custom
func OptWithArg(arg string) command.Applier {
	return command.NewOptionWithArgument("--opt", arg)
}

// OptWithOptionalArgAndDefault("") => --opt=default
// OptWithOptionalArgAndDefault("custom") => --opt=custom
func OptWithOptionalArgAndDefault(arg string) command.Applier {
	if arg == "" {
		arg = "default"
	}
	return command.NewOptionWithArgument("--opt", arg)
}

// OptWithOptionalArgAndNoop("") => nothing will be generated
// OptWithOptionalArgAndNoop("custom") => --opt=custom
func OptWithOptionalArgAndNoop(arg string) command.Applier {
	if arg == "" {
		return command.NewNoopArgument()
	}
	return command.NewOptionWithArgument("--opt", arg)
}

// OptWithRequiredArgAndError("") => error
// OptWithRequiredArgAndError("custom") => --opt=custom
func OptWithRequiredArgAndError(arg string) command.Applier {
	if arg == "" {
		return command.NewErrorArgument(fmt.Errorf("arg is required"))
	}
	return command.NewOptionWithArgument("--opt", arg)
}