Fix stop str merging (#225)

Co-authored-by: Enrique Shockwave <33002121+qeternity@users.noreply.github.com>

python/sglang/lang/interpreter.py

@@ -583,6 +583,10 @@ class StreamExecutor:
         if self.chat_template.stop_str:
             if not clone:
                 clone = self.default_sampling_para.clone()
+            if clone.stop == ():
+                clone.stop = []
+            elif isinstance(clone.stop, str):
+                clone.stop = [clone.stop]
             clone.stop += self.chat_template.stop_str
 
         return clone or self.default_sampling_para