Hi there! Yes it is an offence. If you are caught with too little petrol, you will be told to drive back to Singapore to top-up your petrol, and you can be fined up to $500 for ignoring this rule. The risk of leaving Singapore with low tank is that your car might run out of petrol and you might be stranded in the middle of a massive causeway jam. Nobody wants that right? Therefore, ensure that your tank is at least three-quarter full before heading to Johor.
For more tips on money-saving before planning your trip to Johor, you can read the full article here: https://bit.ly/2z9VYwC