At six weeks, it is still possibly to have an empty gestational sac. The baby may not be developed enough to be seen or have a heartbeat. It could also mean that there is no baby. Another ultrasound is needed to confirm.
If you saw this on a sonogram, it is still probably be fine. A fetus and fetal heartbeat might not appear until after six weeks. Wait a few moere weeks before you allow yourself to become too worried about it.
That will depend on how your doctor dates your pregnancy, if he dated it at five weeks or four weeks then there is no reason for alarm or concern, at six weeks there should be a fetal pole, sac and noticeable tissue in the sac.