Blender Git Loki

Geometry Nodes: Use a default value in the point scale node

This commit adds the ability to provide a default value to
`attribute_try_get_for_output` and uses it for the `Point Scale` node,
which is important because the node uses multiplication.

The idea is to keep "name-specific" functionality in nodes rather than in
the attribute API, otherwise the complexity will be hard to keep track of.
So this fix doesn't apply to the Attribute Vector Math node, but hopfully
that is okay since that's now a lower level node for this purpose anyway.

Differential Revision: https://developer.blender.org/D10115