If the inner door is fine and intact with plastic blocking the microwaves then yes it's safe to use the microwave if the front glass is broken but the inner door is fine.
If the glass is broken and there's nothing in the door to block the microwaves then it's not safe to use the microwave.
But if the inner part of the microwave door is fine then the microwave is safe to use.
If you microwave nothing then the microwaves won't have anything to absorb into which then bounces off the metal inside the microwave and back to the magnetron.
This then damages the microwave and could even start a fire inside the microwave or outside the microwave.
If you've accidentally turned the microwave on for a few seconds with nothing in it then it may still be good to use but any longer than a few seconds of running the microwave with nothing in it increases the risk of damaging the microwave and causing a fire.
When the microwave oven is empty none, or almost none of the microwaves are absorbed.
A large amount of energy reflects around the oven chamber resulting in large standing waves that can damage the unit.
If the microwave works after it has been run while empty, then the unit is safe to use.
Never start an empty microwave.
Without food or a container to absorb the microwaves, the beams are bounced back to the magnetron.
This could damage the machine and may even start a fire.
Microwaves are a form of electromagnetic waves, just like visible light.
The microwaves in the oven disappear when the door opens just as quickly as a room gets dark when you turn off the light.
The microwave oven's job is to cook food; a job it would not do very well if its energy were leaking out into the room.