[bugfix] [cpp-ue4] API often tries to set reference to null when calling GetHttpRetryManager (#10277)

* [cpp-ue4] implemented null check and fatal log for GetHttpRetryManager]

* [cpp-ue4] reimplemented null check as checkf rather than Fatail UE_LOG

* [cpp-ue4] regenerated samples and docs

samples/client/petstore/cpp-ue4/Private/OpenAPIPetApi.cpp

@@ -65,6 +65,7 @@ void OpenAPIPetApi::SetHttpRetryManager(FHttpRetrySystem::FManager& InRetryManag
 
 FHttpRetrySystem::FManager& OpenAPIPetApi::GetHttpRetryManager()
 {
+	checkf(RetryManager, TEXT("OpenAPIPetApi: RetryManager is null.  You may have meant to set it with SetHttpRetryManager first, or you may not be using a custom RetryManager at all."))
 	return *RetryManager;
 }