Fix nullable reference type warnings in FlagsmithConfiguration

Co-authored-by: Claude <noreply@anthropic.com>

Author: emyller

Flagsmith.FlagsmithClient/FlagsmithConfiguration.cs

@@ -1,4 +1,5 @@
-using Microsoft.Extensions.Logging;
+#nullable enable
+using Microsoft.Extensions.Logging;
 using System;
 using System.Collections.Generic;
 using OfflineHandler;