When you check in with the starter (20 minutes or so before your tee time) simply tell him that your playing partner is unable to join you. There are many genuine reasons why someone doesn't turn up - they could be ill or injured, they could be called away in an emergency, they might not like the weather forecast, the tee time might be too early/late, they may have got another time in the ballot, they may have played it before and decide they don't feel the need/want the expense of playing it again. Their absence shouldn’t prejudice your ability to play.