[C++][Pistache] Fix operator== for empty model structs

Created by: kuzkry

When useStructModel=true is set and a model specifies no properties, then a generated struct will not compile.

(...) error: return-statement with no value, in function returning 'bool' [-fpermissive]
 bool NoProperties::operator==(const NoProperties& other) const { return; }

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@ravinikam @stkrwork @etherealjoy @MartinDelille @muttleyxd

P.S. I refuse to run bin/openapi3/cpp-pistache-server-petstore.sh since the changes I get seem to be unrelated.