TT#50701 NGCP::Tempalte::Object: Handle empty peername for self in firstname

When the hostname is self or unknown, we do not return any peername,
so should cope with that to avoid doing comparisons that will end up
emitting warnings.

Change-Id: Ib7e70666991a76b99e1f64ca365a3018320728ec

lib/NGCP/Template/Object.pm

@@ -107,8 +107,9 @@ sub get_firstname
     if (not exists $self->{config}{hosts}{$hostname}) {
         return 'self';
     } else {
+        my @hosts = ($hostname);
         my $peername = $self->get_peername($hostname);
-        my @hosts = ($hostname, $peername);
+        push @hosts, $peername if defined $peername;
 
         return (sort @hosts)[0];
     }