.Net: Added fix for null completion usage details (#4956)

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Resolves: #4953

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Author: dmytrostruk

dotnet/src/Connectors/Connectors.OpenAI/AzureSdk/ClientCore.cs

@@ -1000,6 +1000,16 @@ private static async Task<T> RunRequestAsync<T>(Func<Task<T>> request)
     /// <param name="usage">Instance of <see cref="CompletionsUsage"/> with usage details.</param>
     private void CaptureUsageDetails(CompletionsUsage usage)
     {
+        if (usage is null)
+        {
+            if (this.Logger.IsEnabled(LogLevel.Debug))
+            {
+                this.Logger.LogDebug("Usage information is not available.");
+            }
+
+            return;
+        }
+
         if (this.Logger.IsEnabled(LogLevel.Information))
         {
             this.Logger.LogInformation(