[fix] Notice when ordering some tables in back-end managers

When ajax ordering Plugins, Modules, Content languages, we get a Notice in the php logs.
[03-Apr-2014 08:42:58 UTC] PHP Notice:  Trying to get property of non-object in ROOT/libraries/legacy/model/admin.php on line 1143

It does not prevent the ordering from working fine.

This looks like coming from the fact that these tables are not concerned by content_types and therefore can't have a value for $type->type_alias

Here is a patch