Fix pin conditional

Since the default value of package_apt_pin is an empty string
rather than a boolean, it was failing the conditional test in
Puppet 3.7+. This commit changes the conditional to check for an
empty string.

manifests/repo/apt.pp

@@ -31,7 +31,7 @@
     key_content => $key_content,
   }
 
-  if $pin {
+  if $pin != '' {
     validate_re($pin, '\d\d\d')
     apt::pin { 'rabbitmq':
       packages => 'rabbitmq-server',