add input variables to begin component (#3498)

### What problem does this PR solve?

#3355 

### Type of change

- [x] New Feature (non-breaking change which adds functionality)

api/db/services/dialog_service.py

@@ -98,7 +98,8 @@ def message_fit_in(msg, max_length=4000):
         return c, msg
 
     msg_ = [m for m in msg[:-1] if m["role"] == "system"]
-    msg_.append(msg[-1])
+    if len(msg) > 1:
+        msg_.append(msg[-1])
     msg = msg_
     c = count()
     if c < max_length: