Can I use constant current LED driver as input to Buck regulator?
You can't force a constant current into a buck regulator.  Think about it.  Where would the extra power not used by the final load go?

So this question is really about what the constant current supply will do under low load, and if that is something the buck regulator can handle.  What does the constant current supply do when open?  If it goes to its maximum output voltage, and the voltage is less than the 48 V the buck regulator can handle, then all should be fine.  If it shuts down or occasionally retries with pulses, then it won't work.

As often, the solution is to read the datasheets.