[OpenAI] Suppress null warnings (#24120)

* [OpenAI] Suppress null warnings

* move suppress lines

Author: deyaaeldeen

specification/cognitiveservices/OpenAI.Inference/models/chat.completions.tsp

@@ -156,6 +156,7 @@ model ChatChoice {
     @projectedName("json", "index")
     index: int32;
 
+    #suppress "@azure-tools/typespec-azure-core/no-nullable" "The operation already returns nulls"
     @doc("The reason that this chat completions choice completed its generated.")
     @projectedName("json", "finish_reason")
     finishReason: CompletionsFinishReason | null;

specification/cognitiveservices/OpenAI.Inference/models/completions.create.tsp

@@ -186,11 +186,13 @@ model Choice {
     @projectedName("json", "index")
     index: int32;
 
+    #suppress "@azure-tools/typespec-azure-core/no-nullable" "The operation already returns nulls"
     @doc("The log probabilities model for tokens associated with this completions choice.")
     @projectedName("json", "logprobs")
     @projectedName("csharp", "LogProbabilityModel")
     logprobs: CompletionsLogProbabilityModel | null;
 
+    #suppress "@azure-tools/typespec-azure-core/no-nullable" "The operation already returns nulls"
     @doc("Reason for finishing")
     @projectedName("json", "finish_reason")
     finishReason: CompletionsFinishReason | null;