Support services not specified in metadata.json (#3097)

The commit #3087 introduced
passing a `serviceIdentifier` to the constructor which needs to be
present in the `metadata.json`. For private models thats not the case
and checking for `xmlNoDefaultLists` fails.

Author: workeitel

lib/model/api.js

@@ -34,7 +34,7 @@ function Api(api, options) {
   property(this, 'abbreviation', api.metadata.serviceAbbreviation);
   property(this, 'fullName', api.metadata.serviceFullName);
   property(this, 'serviceId', api.metadata.serviceId);
-  if (serviceIdentifier) {
+  if (serviceIdentifier && metadata[serviceIdentifier]) {
       property(this, 'xmlNoDefaultLists', metadata[serviceIdentifier].xmlNoDefaultLists, false);
   }