Well go and top up the oil - just a bit at a time - leave it to settle for 5 mins - check the oil and top up some more - when its at the correct mark (halfway between the indentations) start it up and listen...... see that it ticks over nice and quiet.
Check the oil everyday for the next week and see if it drops - if it does'nt you should be fine, if it does then get to Nissan to have it checked over.
Hope this helps?
PS check the colour of the oil - is it gluppey black or still clearish?