Fix the condition for upstream members

The condition which decides if we have upstream members configurred or
if we import from PuppetDB was wrong. Even if no upstream members are
defined the condition was true.

Fixes voxpupuliGH-1274

manifests/resource/upstream.pp

@@ -134,7 +134,7 @@
     }),
   }
 
-  if $members != undef {
+  if ! empty($members) {
     $members.each |$member,$values| {
       $member_values = merge($member_defaults,$values,{'upstream' => $name,'context' => $context})