[OpenMP] Don't pass empty files to nvlink

This patch adds and exception to the nvlink wrapper tool to not pass
empty cubin files to the nvlink job. If an empty file is passed to
nvlink it will cause an error indicating that the file could not be
opened. This would occur if the user tried to link object files that
contained offloading code with a file that didnt. This will act as a
workaround until the new OpenMP offloading driver becomes the default.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D117777