A reminder of the question:
But I could not find any information about minimum altitude when go-around should be initiated if the runway is occupied or landing permission is not received
The answer is go-around by the time you cross the threshold if landing clearance is not recieved. Refs are PANS ATM 7.10.2, FAA Order JO 7110.65X 3.10.3, CAP 493 2.1.19, amongst others. I dare say most pilots have continued an approach past the threshold in anticipation that the overworked controller will imminently blurt out your landing clearance. Nonetheless you should understand that the separation minima for a runway does not depend on whether you have touched down, it depends on whether you have passed the threshold.