Allow bundle size to be 0 in clang-offload-bundler

HIP uses clang-offload-bundler to create fat binary. The bundle for host is empty.
Currently clang-offload-bundler checks if the bundle size is 0 when unbundling.
If so it will exit without unbundling the remaining bundles. This causes
clang-offload-bundler not being able to unbundle fat binaries generated for HIP.

This patch allows bundles size to be 0 when clang-offload-bundler unbundles
input files.

Differential Revision: https://reviews.llvm.org/D58057

llvm-svn: 355419