[OPT-4161] CSV export for combined report can be erroneous if not all grouping values show up in all reports

given:
- I have a process with two instances
- Instance A has the variables VA and V1
- Instance B has the variable VB and V1
- Optimize has imported the data
- I create a single report R1 that only shows instance duration A grouped by its variables as table
- I create another report R2 that only shows instance duration B grouped by its variables as table
- I created another report R_C that is a combined report based on R1 and R2
when:
- I download R_C as CSV
then:
- The rows don't match any longer since R1 does not contain variable VB and R2 does not contain variable VA. There are empty lines missing for those lines
expected:
- the CSV file looks like the UI and every possible entry is available for each report column such that all variables that don't exist for the specific report are still present without any value.

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