return null if no vocabulary found

SVN: 27538

datastore_server/source/java/ch/systemsx/cisd/etlserver/registrator/api/v1/impl/SearchService.java

@@ -212,6 +212,6 @@ public class SearchService implements ISearchService
     public IVocabularyImmutable searchForVocabulary(String code)
     {
         Vocabulary vocabulary = openBisService.tryGetVocabulary(code);
-        return new VocabularyImmutable(vocabulary);
+        return (vocabulary == null) ? null : new VocabularyImmutable(vocabulary);
     }
 }