An empty sac could be a very early pregnancy or a kind of abortion where the fetus does not develop. If this is confirmed, then you need to undergo abortion (cleaning of uterus) which can be done medically or surgically or by a combination of the two methods. The choice depends upon various factors including how big your uterus is, whether or not you have medical problems, allergy to certain drugs etc.
The pregnancy test (if done on urine) can be a false negative if the sensitivity of the kit is not high, or the kit has not be stored at the correct (recommended) temperature. It can also be negative if you had an ectopic pregnancy (pregnacy outside uterus) which has been disturbed (which, according to the ultrasound report is not so).
It is best to correlate the ultrasound findings with the clinical findings an the blood beta HCG test. Your obstetrician will be the best person to guide you. Too many opinions (without examination) can, at times, do more harm than good.