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 point scale 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. Note that this fix doesn't
apply to the Attribute Vector Math node, but that's hopefully okay since that's now
a lower level node for this purpose anyway.