Suppress undef warnings in example receiver (thanks to DanS)

src/warden-client/doc/example-receiver.pl.txt

@@ -38,7 +38,7 @@ print "+------------------------------------------------------------------------
 while (my @new_events = WardenClientReceive::getNewEvents($warden_path, $requested_type)) {
   foreach my $event_ref (@new_events) {
     my @event = @$event_ref;
-    print "| " . join(' | ', @event) . " |" . "\n";
+    print "| " . join(' | ', map { $_ || '' } @event) . " |" . "\n";
   }
 print "+------------------------------------------------------------------------------------------------------------------------------------------+\n";
 }