Don't add ACL when hosts array is empty

Varnish doesn't allow empty ACLs. Currently, the module adds two empty ACLs by default: `blockedips` and `purge`. This PR is one possible way of solving this problem: by ignoring any ACLs that contain no hosts. 

Alternatively, we can change the `$default_acls` in `vcl.pp` to only add `blockedips` and `purge` when  `$purgeips` and `$blockedips` contain at least one element. 

Which solution do you prefer?