allow for the get_default_version to not return a version

that should not happen, but non-species database will not have a version
this includes ancestral sequences databases

modules/Bio/EnsEMBL/DBSQL/CoordSystemAdaptor.pm

@@ -641,8 +641,6 @@ sub get_default_version {
     }
   }
 
-  throw('No default version is defined') if(!$version);
-
   return $version;
 }