[NVPTX][AMDGPU][CodeGen] Fix `local_space nullptr` handling for NVPTX and...

[NVPTX][AMDGPU][CodeGen] Fix `local_space nullptr` handling for NVPTX and local/private `nullptr` value for AMDGPU. (#78759)

- Address space cast of nullptr in local_space into a generic_space for
the CUDA backend. The reason for this cast was having invalid local
memory base address for the associated variable.
- In the context of AMD GPU, assigns a NULL value as ~0 for the address
spaces of sycl_local and sycl_private to match the ones for opencl_local
and opencl_private.