Throw compilation error when using collect hit policy with aggr. function and multiple outputs

According to the DMN spec., "Decision tables with compound outputs support only the following hit policies: Unique, Any, Priority, First, Output
order, Rule order and Collect without operator, because the collect operator is undefined over multiple outputs."

However when I use decision table with "COLLECT" hit policy, "sum" aggregate function and multiple outputs, the table compiles. When evaluating the table, I get nulls as output value results, but no error message in DMNResult.

I will provide a reproducer in a PR.