How can the armature current of a DC shunt motor be zero at no-load?
In 'imaginary world' the motor is armature is rotating freely without bearing friction or windage, so once it get up to speed (which takes forever, but it gets very close very quickly) it continues to rotate at exactly the RPM to counter the applied voltage. Since there is zero current, zero torque is produced (but none is needed to keep the rotor spinning).
If you subsequently reduce the voltage, the current flows in the opposite direction compared to starting up, and the rotor RPM drops. If you increase the voltage the motor RPM increases to balance the applied voltage with back EMF, as before.
In reality, the no-load current is not zero, but it's a lot less than the current at full load and much, much less than the current with rotor locked.