Don't use sysroot/include when sysroot is empty.

Baremetal toolchain add Driver.SysRoot/include to the system include
paths without checking if Driver.SysRoot is empty. This resulted in
"-internal-isystem" "include" in the command. This patch adds check for
empty sysroot.

Reviewed By: jroelofs

Differential Revision: https://reviews.llvm.org/D92176