make test for existence of ::packer_build_name work for Puppet 4+

manifests/wallet.pp

@@ -72,7 +72,7 @@ define base::wallet(
         file { $path: ensure => absent }
     }
     'present': {
-      if ($build_ok or !$::packer_build_name) {
+      if ($build_ok or (!defined("$::packer_build_name"))) {
         case $type {
           'keytab': {
             base::wallet::keytab { $name: