Aria2c: Fallback to an empty list if stopped_downloads is None

This was fine during most testing in the `for` loop below it, but there's also a len() a bit below that.

devine/core/downloaders/aria2c.py

@@ -212,8 +212,8 @@ def download(
                 secret=rpc_secret,
                 method="aria2.tellStopped",
                 params=[0, 999999]
-            )
-            for dl in stopped_downloads or []:
+            ) or []
+            for dl in stopped_downloads:
                 if dl["status"] == "complete":
                     yield dict(advance=1)
                 elif dl["status"] == "error":