transforms/lower_units: support empty list

artiq/transforms/lower_units.py

@@ -89,6 +89,7 @@ class _UnitsLowerer(ast.NodeTransformer):
 
     def visit_List(self, node):
         self.generic_visit(node)
+        if node.elts:
             us = [getattr(elt, "unit", None) for elt in node.elts]
             if not all(u == us[0] for u in us[1:]):
                 raise units.DimensionError