Fix os_family and distribution on archlinux

Since `1.7.0` the facts `os_family` and `distribution` are set to `NA` on Archlinux.
I think this was introduced by https://github.com/ansible/ansible/commit/f4fd9d36.
The file `/etc/arch-release` is empty on arch which is the reason why it is ignored.

This was originally pullrequested in #9012 which was closed because repositories were reorganized.