[C++][Pistache] Add neutral discards to supress a potential -Wunused-parameter warning

Created by: kuzkry

Note: this is dependent on #5124, so there is only one commit in this PR.

When useStructModel=true is set and a model specifies no properties and assuming no compilation errors, then a generated struct will not produce -Wunused-parameter warnings.

Unfortunately, I didn't find a nice and clean solution. My ideas include:

• (void) argument or static_cast<void>(argument)
• conditionally specify only type like const NoProperties& instead of const NoProperties& o, however that means a messier template
• __attribute__((unused)) but this isn't standard
• [[maybe_unused]] but it's C++17 so I guess it's a no-go
• template <typename T> void ignore(T) {}

So I decided to use the first option as it's a noop for compilers but I dunno. If you have any better idea, let me know.

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• Read the contribution guidelines.
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@ravinikam @stkrwork @etherealjoy @MartinDelille @muttleyxd

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