NULL

From GMod Wiki

NULL is an entity object that does not exist in the world.

• GetPhysicsObject can return NULL
• Getting an entity off a non existant entity index can return NULL
• Creating an entity with a class that does not exist also can return NULL

Remember that in addition to being NULL, a value referring to an inexistant entity can also be nil, which must be treated separately.

If you are checking whether an entity exists or not then use either ValidEntity(ent) or this condition "Ent and Ent:IsValid()". First check if it's nil then if it's valid.