mkbootfs: Allow invocation with no directories specified

1. A non-empty archive can still be generated, if a dev nodes
file is specified; and
2. cpio itself handles the case of an empty archive
(containing only the trailer entry), for example:
  echo -n | cpio -oc | cpio -it

Bug: 328362894
Change-Id: If17796f445e50842249554a1e82478bc375cf5eb