[cpp-ue4] add support for empty response

Created by: Falanwe

Very simple change to the api template for cpp-ue4 to handle an API that respond without any content (for instance if it responds with a single 200 to indicate success).

Sadly, there are no test config for cpp-ue4 (yet?). I tested the change directly inside the UE4 project we have in which we are using openapi-generator (as it's a proprietary game I cannot share it obviously, but the resulting code did compîle well).

@ravinikam @etherealjoy @MartinDelille @muttleyxd @Kahncode

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added Client: C++ Enhancement: General labels

closed

changed milestone to %5.0.0