`IGPUDescriptorSet::validateWrite` shouldn't crash if desc is nullptr

[Erfan-Ahmadi]

I think one of the responsibilities of the validateWrite function is, well, to validate :D
I encountered a scenario where I was trying to write a nullptr image (failed image creation) to the set and it crashed in IGPUDescriptorSet::validateWrite in this line

I believe there needs to be a null check in there as well.