[HOTFIX]: Avoid to assign None value to self.api_key in dashscope cha…

…t model wrapper (#335)

src/agentscope/models/dashscope_model.py

@@ -58,7 +58,8 @@ def __init__(
         self.generate_args = generate_args or {}
 
         self.api_key = api_key
-        dashscope.api_key = self.api_key
+        if self.api_key:
+            dashscope.api_key = self.api_key
         self.max_length = None
 
         # Set monitor accordingly