Fix Sphinx results order for keywordless queries.
Fix Sphinx results order for keywordless queries.

Refs #475.

Tatoeba/tatoeba2,Tatoeba/tatoeba2,Tatoeba/tatoeba2,shashank19gaurav/tatoeba2,shashank19gaurav/tatoeba2,shashank19gaurav/tatoeba2,Tatoeba/tatoeba2,shashank19gaurav/tatoeba2,Tatoeba/tatoeba2