Animation Channel's "Node" Member Isn't Required

[cwfitzgerald]

Per https://registry.khronos.org/glTF/specs/2.0/glTF-2.0.html#animations:

When node isn’t defined, channel SHOULD be ignored. Valid path names are "translation", "rotation", "scale", and "weights".

Blockbench in particular seems pretty happy at spitting out non-targetted channels. This causes a json parsing error, as the gltf crate expects there to be a node, but one isn't present.