fix: only add toolCalls when it's not empty

Author: alexcheng1982

src/main/kotlin/com/javaaidev/openai/OpenAIChatModel.kt

@@ -100,7 +100,9 @@ class OpenAIChatModel(
                             )
                             .build()
                     }?.let {
-                        messageParamBuilder.toolCalls(it)
+                        if (it.isNotEmpty()) {
+                            messageParamBuilder.toolCalls(it)
+                        }
                     }
                     paramsBuilder.addMessage(messageParamBuilder.build())
                 }