common.yaml

---
sites:
  - domain: 'www.example.com'
  - domain: 'www2.example.com'
    ip: '1.1.1.1'

output.log

[root@master ~]# puppet apply test.pp
Notice: Compiled catalog for master in environment production in 0.12 seconds
Notice: www.example.com has no ip
Notice: /Stage[main]/Main/Site[{"domain"=>"www.example.com"}]/Notify[www.example.com has no ip]/message: defined 'message' as 'www.example.com has no ip'
Notice: www2.example.com on 1.1.1.1
Notice: /Stage[main]/Main/Site[{"domain"=>"www2.example.com", "ip"=>"1.1.1.1"}]/Notify[www2.example.com on 1.1.1.1]/message: defined 'message' as 'www2.example.com on 1.1.1.1'
Notice: Finished catalog run in 0.18 seconds

test.pp

define site() {
  $domain = $title['domain']
  $ip = $title['ip']

  if $ip {
    notify { "${domain} on ${ip}": }
  }
  else {
    notify { "${domain} has no ip": }
  }
}

$sites = hiera('sites')
site { $sites: }