6.4mm Gestational Sac empty

I had an early pregancy scan lastnight, it has been predicted I am 6W4D. The first day of my last period was the 7th May. My period was 9 days long, I have PCOS and have irregular periods. I had a transvaginal scan and it was hard to locate my gestational sac, eventually it was found, it was 6.4mm and empty. I have another scan booked in for the 6th July. The midwife said it could be too early to see or that my dates could be earlier than expected. I took a pregnancy test on the 1st June and that's when I had my first very faint positive. I am trying to work out the timeframe but it seems to me like I would be 6/7 weeks. Has anyone experienced this and still had a healthy pregnancy?