Satoshi-san,
I don't believe no load operation is touched on in the data sheet and if it is the converter they are using has sync rectifiers on the output which prevents the output voltage build up that diodes allow.
The discription i furnished is what is happenning and the data sheet specifies 0% minimum duty cycle (pulse skipping) and it specifies a PWM offset of 0.43 to 0.60 volts so any time the FB signal is below the (PWM offset x 5) there will be no pulses.
John