Redmine/6119

The case you describe is not an argument for having a "value that is not a value" - it's an argument for using a list variable (which can be empty, or supply a value, or even supply several) and interpolating it:
```
class1::
   "var" slist => { "mybundle" };
 !class1::
   "var" slist => {};

  any::
    bundlesequence => { @(var), "test" };
```
(Note on replacing class2 with !class1: in your version, what should happen if neither or both of class1 and class2 is defined ?  I would have a separate sub-list for each class and concatenate the various lists into the bundlesequence.)

If you maybe want to supply a value and maybe don't, a list is the natural answer; it can have no entries, it can have one (and indeed it could have more, if you wanted that).