The output node types were seemingly incorrect since the initial commit
and would produce incorrect MaterialX documents[1]. When MaterialX is
upgraded to 1.38.10 this will now fail material creation in Hydra.
Use the node type directly instead of setting things to their default
`color3` output type.
[1] See PR for an example.
Pull Request: https://projects.blender.org/blender/blender/pulls/132362
95be928c5e