This helps to tell when a pointer is expected to be null, and avoid
overly verbose code when dereferencing. This commit also includes
a few other cleanups in this area:
- Use const in a few places
- Use `float2` instead of `float[2]`
- Remove some unnecessary includes and old code
The change can be continued further in the future.
This just moves a couple of files in `space_node` to C++ and fixes
related errors.
The goal is to be able to use C++ data structures to simplify the code.
Differential Revision: https://developer.blender.org/D11451
