Adds failing unit-test for issue #4839 NPE when projecting to an inte…

[fdutton]

…rface and using StringBasedAggregation

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[christophstrobl]

thank you @fdutton - it seems the mail adr. used in the commit is not linked to your account.

[christophstrobl]

Thank you @fdutton - issues has been resolved, test has been merged.