Fix Propagate Positions

lark/parse_tree_builder.py

@@ -25,9 +25,7 @@ class PropagatePositions:
     def __call__(self, children):
         res = self.node_builder(children)
 
-        if isinstance(res, Tree) and getattr(res.meta, 'empty', True):
-            res.meta.empty = True
-
+        if isinstance(res, Tree):
             for c in children:
                 if isinstance(c, Tree) and c.children and not c.meta.empty:
                     res.meta.line = c.meta.line

lark/tree.py

@@ -8,7 +8,8 @@ from copy import deepcopy
 
 ###{standalone
 class Meta:
-    pass
+    def __init__(self):
+        self.empty = True
 
 class Tree(object):
     def __init__(self, data, children, meta=None):