Prevent NPE when delta is null in Azure OpenAI streaming

Azure OpenAI sends streaming chunks without delta content for
end-of-stream markers and content filter responses. Added null
check to prevent NullPointerException in streaming handler.

Author: Vincenzo Sciangola

model-providers/openai/azure-openai/runtime/src/main/java/io/quarkiverse/langchain4j/azure/openai/AzureOpenAiStreamingChatModel.java

@@ -234,6 +234,9 @@ private static void handle(ChatCompletionResponse partialResponse,
             return;
         }
         Delta delta = choices.get(0).delta();
+        if (delta == null) {
+            return;
+        }
         String content = delta.content();
         if (content != null) {
             handler.onPartialResponse(content);