back-end: Minor fix related to phrase handling

Fixes passing an empty string to the db in (probably) rare cases.

back_end/src/json_handlers.rs

@@ -56,10 +56,13 @@ fn handle_pairing_request(root: Value, tx: SyncSender<DBHandlerRequest>) -> Resu
                         phrase_str = phrase_str.split_at(idx).0;
                     }
                 }
+
+                if !phrase_str.is_empty() {
                     phrase = Some(phrase_str.to_owned());
                 }
             }
         }
+    }
     if tx
         .send(DBHandlerRequest::GetID {
             response_sender: player_tx,