`GVolumeGrid` and `VolumeGrid<T>` are basically pointer wrappers.
Assigning a nullptr is possible and allowed by code, but node socket
code doesn't currently check if a grid pointer is null before accessing
it.
Disallow null grid pointers in socket values. Asserts are added for this
purpose. The Points-to-SDF node checks the grid value before writing to
output.
Pull Request: https://projects.blender.org/blender/blender/pulls/122996
ddd079b54f