Fixed a pack test that was not compatible with storages that always

return an object count of 0.

NEWS.txt

+Next Release
+===========
+
+Bugs Fixed:
+
+- Fixed a pack test that was not compatible with storages that always
+  return an object count of 0.
 
 Whats new in ZODB 3.8.1
 =======================

src/ZODB/tests/PackableStorage.py

@@ -326,7 +326,8 @@ class PackableStorage(PackableStorageBase):
         root[2] = conn.get_connection('o').root()
         transaction.commit()
         db.pack(time.time()+1)
-        assert(len(self._storage) == 1)
+        # some valid storages always return 0 for len()
+        self.assertTrue(len(self._storage) in (0, 1))