fix

Author: christothes

src/Custom/Chat/OpenAIChatModelFactory.cs

@@ -69,6 +69,7 @@ public static ChatCompletion ChatCompletion(
         contentTokenLogProbabilities ??= new List<ChatTokenLogProbabilityDetails>();
         refusalTokenLogProbabilities ??= new List<ChatTokenLogProbabilityDetails>();
         messageAnnotations ??= new List<ChatMessageAnnotation>();
+        contentParts ??= new List<ChatMessageContentPart>();
 
         InternalChatCompletionResponseMessage message = new(
             content: content,