Adjust BuildQuery param types for .NET Framework compatibility

Use conditional compilation to set BuildQuery's tuple Value type
to object for .NET Framework and object? for other targets,
ensuring compatibility with nullable reference types across
different .NET versions.

src/ReC.Client/ReCClient.cs

@@ -33,7 +33,11 @@ namespace ReC.Client
             _http = httpClientFactory.CreateClient(ClientName);
         }
 
+#if NETFRAMEWORK
         private static string BuildQuery(params (string Key, object Value)[] parameters)
+#else
+        private static string BuildQuery(params (string Key, object? Value)[] parameters)
+#endif
         {
             var parts = parameters
                 .Where(p => p.Value != null)