Apply ruff/flake8-return rule RET501

RET501 Do not explicitly `return None` in function if it is the only possible return value

Author: DimitriPapadopoulos

src/zarr/abc/store.py

@@ -170,7 +170,7 @@ async def _set_many(self, values: Iterable[tuple[str, Buffer]]) -> None:
         Insert multiple (key, value) pairs into storage.
         """
         await gather(*(self.set(key, value) for key, value in values))
-        return None
+        return
 
     @property
     @abstractmethod