After the point distribute node, reading scale for the first time doesn't have any data, so if you try to multiply the value will be zero.

1. Add a new nodes modifier on the default cube
2. Add the following node setup with an instancing node afterwards.
3. See that instances have zero scale unless the attribute fill node is used.

The fix is to have a map of attribute names to default values. In this case the map would be
scale -> [1.0, 1.0, 1.0]