For legacy images (images with a CUDA_VERSION set but no CUDA_REQUIRES set), the
default behaviour for device envvars is to treat non-existence as all.
This change ensures that the NVIDIA_IMEX_CHANNELS envvar is not treated in the same
way, instead returning no devices if the envvar is not set.
Signed-off-by: Evan Lezar <elezar@nvidia.com>