Fix vendorsetup.sh inclusion for zsh

zsh defaults to the NOMATCH option when a glob doesn't match any file.
This causes the inclusion of the vendorsetup.sh files in envsetup.sh to
fail if any argument to ls won't expand. Setting NULLGLOB, which
resembles bash's behavior, for the subshell fixes that.

Change-Id: I549840e3b1ed17b76dbcd94fdd63321ec5915162