Your employer does not require you any more that is why they ask you to resign instead of terminating the services.
The resignation is considered as the self decision, so there is no legal complicacy for the company. But in the case of termination the person can take a legal course against termination.
You decide what is best in this juncture,(1) if you are considering the charge is wrong then do not resign (2) if there is some truth in the charges it is better to resign but discuss with your manager to allow some more time or ask them to pay the notice amount against your resignation (3) if you think there is low chance to get a new job in this crisis, so leave the matter to your employer what they want to do with you without your resignation.