sweep_artifacts: narrow the optional flow store for mypy

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WgBfkm4rCmtd1T4vHWjEPx

backend/fluksio/flow/runs.py

@@ -403,14 +403,15 @@ def sweep_artifacts(
             return 0
         keep = set(session.exec(select(col(RunArtifact.digest)).distinct()).all())
 
-    for name in flows.list_flows() if flows else ():
-        for draft in (False, True):
-            try:
-                flow = flows.read_flow(name, draft=draft)
-            except FlowNotFound:
-                continue
-            for declared in flow.inputs:
-                _references_in(declared.initial, keep)
+    if flows is not None:
+        for name in flows.list_flows():
+            for draft in (False, True):
+                try:
+                    flow = flows.read_flow(name, draft=draft)
+                except FlowNotFound:
+                    continue
+                for declared in flow.inputs:
+                    _references_in(declared.initial, keep)
 
     keys = state.keys()
     for start in range(0, len(keys), _SWEEP_BATCH):