[tests][Driver] Pass an empty sysroot for `DEFAULT_SYSROOT` builds

The baremetal-sysroot test fails when the toolchain is configured with
DEFAULT_SYSROOT. So, to emulate not having passed one at all, let's
pass an empty sysroot instead.

https://reviews.llvm.org/D119144

Patch by Carlo Cabrera <carlo.antonio.cabrera@gmail.com>