Fix build error on empty ramdisk

package `init_vendor` is the only content of ramdisk so far.
We would get build error if we do not include init_vendor.
The patch fix the build error for the case that ramdisk is empty.

Bug: 129386309
Test: lunch aosp_arm64_ab-userdebug; make ramdisk -j
Test: Build pass
Change-Id: I7c7c828b5f29350268d4789393b90740dd68162d