Empty sort_key_string should be fixed to prevent failures

This issue is derived from EZP-22308 and EZP-23894.

Basically, due some known bugs, it's possible that sort_key_string is left empty what may have effects in searches trough the API, for example.

Even the known bugs that could leave empty sort_key_strings being already fixed, the sort_key_string fields will remain empty what will result in problems anyway.

That said, it's important to handle somehow the problem of empty sort_key_string, being trough some kind of fix in the code or and update script.