[C++][Pistache] Compile error when nesting references (#16711) (#18586)

When components/schema/<object> reference other objects, validate() was
getting called on the referenced objects with no arguments. The return
value was void, but checked as if it was a boolean value.

samples/server/petstore/cpp-pistache-nested-schema-refs/impl/StoreApiImpl.cpp

@@ -0,0 +1,35 @@
+/**
+* Test swagger file
+* No description provided (generated by Openapi Generator https://github.com/openapitools/openapi-generator)
+*
+* The version of the OpenAPI document: 1.0.0
+* 
+*
+* NOTE: This class is auto generated by OpenAPI Generator (https://openapi-generator.tech).
+* https://openapi-generator.tech
+* Do not edit the class manually.
+*/
+
+#include "StoreApiImpl.h"
+
+namespace org {
+namespace openapitools {
+namespace server {
+namespace api {
+
+using namespace org::openapitools::server::model;
+
+StoreApiImpl::StoreApiImpl(const std::shared_ptr<Pistache::Rest::Router>& rtr)
+    : StoreApi(rtr)
+{
+}
+
+void StoreApiImpl::get_nested_object(Pistache::Http::ResponseWriter &response) {
+    response.send(Pistache::Http::Code::Ok, "Do some magic\n");
+}
+
+}
+}
+}
+}
+