fix: Fixed an issue where the first message would be displayed when sending the second message #2625 (#2626)

### What problem does this PR solve?

fix: Fixed an issue where the first message would be displayed when
sending the second message #2625

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)
- [ ] New Feature (non-breaking change which adds functionality)
- [ ] Documentation Update
- [ ] Refactoring
- [ ] Performance Improvement
- [ ] Other (please describe):

api/apps/conversation_app.py

@@ -37,7 +37,9 @@ from graphrag.mind_map_extractor import MindMapExtractor
 def set_conversation():
     req = request.json
     conv_id = req.get("conversation_id")
-    if conv_id:
+    is_new = req.get("is_new")
+    del req["is_new"]
+    if not is_new:
         del req["conversation_id"]
         try:
             if not ConversationService.update_by_id(conv_id, req):
@@ -56,7 +58,7 @@ def set_conversation():
         if not e:
             return get_data_error_result(retmsg="Dialog not found")
         conv = {
-            "id": get_uuid(),
+            "id": conv_id,
             "dialog_id": req["dialog_id"],
             "name": req.get("name", "New conversation"),
             "message": [{"role": "assistant", "content": dia.prompt_config["prologue"]}]