Does an induction cooktop consume as much energy when it's heating something as it does when it isn't? If you plug in an electrical charger or an adapter of an electronic device such as a mobile phone or a laptop into the power socket, but you don't actually attach the device into the charger, the charger does not consume significantly as much electrical energy as it would if the device were connected and drawing power.
Is it the same way for an induction cooktop? Does it also only consume significant amount of energy when a cookware is actually on top of it and getting heated?
 A: Induction cooktops contain electromagnets below each pot or pan station.  When a station is switched on, electric current flows through wire wrapped around an iron core.  In order for magnetic flux to be induced in the iron core, the electric current must constantly change, so the current must alternate.  The iron core concentrates the magnetic flux generated by the electric current, and a magnetic field is created on the cooktop.  
If you place a ferromagnetic pot or pan above the electromagnet, the changing magnetic field induces electric current in the pot or pan.  The induced current swirls around the pot or pan and dissipates its energy against the electrical resistance of the pot in the form of heat.
If there is no pot or pan on top of the electromagnet, no electric current will be induced,  but the electromagnetic field will still exist, and the circuit will still draw current according to Ohm's law
$$I = V / R$$
where $I$ is current, $V$ is potential difference across the circuit, and $R$ is resistance.
Induction cooktops must have time changing current in order to generate a magnetic field, so the simple Ohm's law must be altered to include capacitance and inductance.  The result is that resistance is replaced by impedance.
Nevertheless, the idea is the same, and even without inducing electric current above it, the electromagnet will draw current which is directly proportional to the voltage input, and indirectly proportional to the impedance.  The amount of current drawn by the electromagnet when there is no pot being heated will be minimal, as Daniel pointed out in his answer. 
Induction cooktops have automatic circuits which shut off the electromotive force to a cooking station if there is no pot or pan on it.  But in the absence of such a shut-off, current would continue to be drawn, though an insignificant amount with no pot being heated.
A: I disagree with the other two answers, or at least believe they are not very clear. 
The important point is that, as you suggested, a induction cooker only consumes significant amounts of energy when a pot is actually on top of it.
There are, of course other losses, but without any metal object in the vicinity, the cooktop is like a transformer without a load, which (in the ideal case) does not dissipate any energy. 
A: I believe an induction cooktop works by creating an alternating magnetic field, which heats any ferromagnetic materials close to the inductor.
If you have an inductor switched on, but you're not heating anything, the inductor is still producing a magnetic field. It takes energy to sustain such a field, even if there's nothing being heated. 
Also, even if the cooktop inductor does not appear to get hot, any wire with current passing through it gives off heat energy. The amount of energy given off depends on the amount of current, and the material the wire is made from.
So no, it would consume energy as long as it is switched on.
Edit:
Without a pot/pan or other ferromagnetic material near the inductor, it will consume significantly less energy than it would while actively heating a pan. 
It also appears from Ernie's answer that that type of cooktop can have an automatic switch-off that I did not know about. If it does indeed have a switch-off, then it would be safe to assume that as long as it is functional, the cooktop will not be using large amounts of energy.
*Note:
Even with the switch-off, if it has one, it would be inadvisable to rely on it to turn your stove off for you. You should always turn your stove completely off before leaving it unattended, even if it used very little energy without a pot on it. That said, I doubt that was the point of your question.
