Dry Oil Filter on Mazda 323

This question is about a Mazda 323 sedan that has driven 50,000kms. The car has been well maintained since new and receives regular servicing. On doing a recent oil change I found the oil filter was completely dry - it contained no oil at all. The oil light on the instrument panel is functioning normally - it comes on when the ignition is turned on and goes off immediately when the engine starts. The car is running well and there are no unusual smells or noises. There are no oil leaks. Can you tell me why the oil is not flowing through the oil filter.