the activity observer doesn't get shown empty buckets

so don't expect to see empty lists in the tests

src/_zkapauthorizer/tests/test_lease_maintenance.py

@@ -642,6 +642,7 @@ class MaintainLeasesFromRootTests(TestCase):
                 except KeyError:
                     continue
                 else:
+                    if shares:
                         expected.append(list(stat.size for stat in shares.values()))
 
         # The visit order doesn't matter.