similarly to the changes for process reports (done in
OPT-2503) it should be implemented for decision reports input/output variables as well to be consistent.
Input variables should be checked if it's possible that undefined variables exist, but if yes, then it should be implemented.
- it is possible to filter any variable type for null/undefined
- report groupby variable returns an additional bucket with the aggregated result of decision instances with undefined/null value for the grouped variable
- the rest api is updated