beam: fix LRU cache get() missing-key return (None -> -1 to match spec/tests)

wizard-work/beam/lru_cache.py

@@ -66,7 +66,7 @@ class LRUCache:
         with self._lock:
             node = self._map.get(key)
             if node is None:
-                return None if default is _MISSING else default
+                return -1 if default is _MISSING else default
             self._promote(node)
             return node.value