Don't assume config.modules_to_save to be iterable

src/peft/peft_model.py

@@ -968,7 +968,7 @@ def delete_adapter(self, adapter_name: str) -> None:
     def modules_to_save(self) -> Optional[set[str]]:
         modules: set[str] = set()
         for config in self.peft_config.values():
-            if hasattr(config, "modules_to_save"):
+            if getattr(config, "modules_to_save", None) is not None:
                 # modules_to_save can only be a sequence of str, not a str
                 modules.update(config.modules_to_save)