Relevant dc motor specs for buck converter design
I don't understand how the output current would vary when supplying
  less than 24v to the motor (since the voltage supplied to the motor
  could be as low as 15v). My intuition is that the current draw would
  be greater


At lower voltages the DC motor current will tend to be proportionally less unless you anticipate a stall situation.

If the motor spec says 50 mA is the maximum no load current at 24 volts then (ignoring mechanical loads on the armature that you may have), I'd assume it could be typically about 30 mA and, at 15 volts (providing you don't suffer a motor stall), the current will be about 20 mA.

So, if you want CCM operation then aim for a minimum load current of 15 mA to have some degree of margin.

However, a more detailed examination of the motor's data sheet might give typical figures that you can use OR just test the motor on a DC supply.

Alternatively, operate at high enough PWM frequencies so that ripple will be naturally low and just design it for DCM operation. 

OR design a synchronous buck converter (more efficient) and let the inductor current flow negative on light loads because you will still have decent ripple using a synchronous buck converter and control will still be quite close to Vout = Vin x duty cycle.