Optional attributes in ACS workflows

Hello!

I'm trying to build reusable workflow templates in Adobe Campaign Standard that can handle variable input structure (without having to reconfigure certain workflow steps every time the input changes).

For example, in the the Extract file step I want to be able to cope with optional attributes (that may or may not be present on the input transition), and default them to empty string for the output.

I thought that an expression like this - for the missing 'optionalAttribute' - could handle the attribute that is not always present, and write out an empty string:

Case(When(NULL != optionalAttribute, optionalAttribute),Else(""))

But the workflow fails with the following error:

- 'optionalAttribute' element unknown

I understand, that to ensure consistency, we should always configure a fixed structure for input files and transitions, but being able to handle the case like above would make our workflows easier to reuse.

Is it possible to write an expression for e.g. the Extract file step, that handles the case above?

Thanks!