CI: build_changed_aports: fix checksum for 0 pkgs
When using '[ci:skip-build]' without any changed packages, do not attempt to run "pmbootstrap checksum" without any package arguments. This may happen, because CI in MRs runs across all patches in the MR, but CI on branches (master, v20.05) only run on the last commit.
Prevents CI failures like this one: 9a48b41a