ShaderInput lookup used to be explicit as in we know for sure that the
shader input exists. This was guarded by a assert. During development
of the graphics pipeline we came to the conclusion that this isn't
always the case due to its late bindings of resources.
This PR Makes the shader input lookup return an optional result.
This result can then be checked depending on the area where it is used
to validate the existence.
Pull Request: https://projects.blender.org/blender/blender/pulls/107735
a0ea40034b