At present we fail in two ways if we try to create empty RPMs:

- we expect srcs to be both non-empty and will fail if it is
  empty and we have no spec file

- we don't emit anything for the `%files` block in the RPM if
  there are no actual files and rpmbuild doesn't like this

This change tweaks the former condition so that srcs has to be
non-None or we have to have a specfile, but will allow us to have an
empty (`[]`) value for srcs.  Additionally, it injects
`%defattr(-,root,root)` as a reasonable default for the `%files`
blocks so as to allow rpmbuild to be happy with what we're providing.

We should be safe to inject this unconditionally instead of special
casing on whether or not we have no actual files.