MT#61052 NGCP::Template::Plugin::Utils: Do not generate bogus YAML on empty string

When we pass an empty string the current to_yaml() function generates
broken YAML such as "--- ''", which then breaks code parsing that
expecting valid YAML. If we pass an empty string, return an empty string
as well.

Change-Id: I010cd9d544bd505446f0016ecc6302a987921570

lib/NGCP/Template/Plugin/Utils.pm

@@ -40,6 +40,7 @@ sub to_config_general {
 
 sub to_yaml {
     my ($self, @params) = @_;
+    return q{} unless $params[0];
     return YAML::XS::Dump($params[0]);
 }