Fixes null reference exception. Closes #1342

Author: waldekmastykarz

src/Microsoft.OpenApi.Hidi/OpenApiService.cs

@@ -151,7 +151,7 @@ private static OpenApiDocument ApplyFilters(HidiOptions options, ILogger logger,
             }
             else if (postmanCollection != null)
             {
-                requestUrls = EnumerateJsonDocument(postmanCollection.RootElement, requestUrls);
+                requestUrls = EnumerateJsonDocument(postmanCollection.RootElement, new());
                 logger.LogTrace("Finished fetching the list of paths and Http methods defined in the Postman collection.");
             }