fix bug with undef vals in hosts struct

Netdisco/lib/App/Netdisco/Util/DNS.pm

@@ -103,11 +103,10 @@ addresses which resolved.
 
 sub hostnames_resolve_async {
   my $ips = shift;
-
   my $resolver = AnyEvent::DNS->new();
 
   my %HOSTS = ();
-  $HOSTS{$_} = [ map { [ format_address $_->[0] ] }
+  $HOSTS{$_} = [ map { [ $_ ? (format_address $_->[0]) : '' ] }
                      @{$AnyEvent::DNS::EtcHosts::HOSTS{$_}} ]
     for keys %AnyEvent::DNS::EtcHosts::HOSTS;