Concentration with no object?
It means to incline your mind towards dispassion, dispassion towards the sense objects, dispassion towards craving for existence and dispassion towards craving for non-existence.
Inclining your mind toward dispassion means to incline your mind towards disenchantment, no longer being enchanted with sense objects, craving for existence and craving for non-existence.
To incline your mind towards disenchantment, you must see reality as it is by observing any/all of the three marks of existence:

changing nature of reality
selfless nature of reality
unsatisfactory nature of reality

By observing these qualities of existence, the mind gradually loses its interest in grasping to such unreliable things for a source of lasting happiness, thus inclining towards disenchantment, thus inclining towards dispassion, thus inclining towards Nibbana.

Does this mean concentration with no object?

No - It means concentration of any object while noticing its changing nature, selfless nature and/or unsatisfactory nature.