[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.h

@@ -0,0 +1,52 @@
+/**
+* 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.
+*/
+
+/*
+* StoreApiImpl.h
+*
+* 
+*/
+
+#ifndef STORE_API_IMPL_H_
+#define STORE_API_IMPL_H_
+
+
+#include <pistache/endpoint.h>
+#include <pistache/http.h>
+#include <pistache/router.h>
+#include <memory>
+#include <optional>
+
+#include <StoreApi.h>
+
+
+#include "OuterType.h"
+
+namespace org::openapitools::server::api
+{
+
+using namespace org::openapitools::server::model;
+
+class  StoreApiImpl : public org::openapitools::server::api::StoreApi {
+public:
+    explicit StoreApiImpl(const std::shared_ptr<Pistache::Rest::Router>& rtr);
+    ~StoreApiImpl() override = default;
+
+    void get_nested_object(Pistache::Http::ResponseWriter &response);
+
+};
+
+} // namespace org::openapitools::server::api
+
+
+
+#endif