Do we say that Allah did not predestine the occurrence of something that did not occur, or that Allah predestined its non-occurrence?

The summary of the answer: This issue hinges on the premise that non-existence is not a thing, and therefore, it does not require an agent. The agent only affects existent matters. Thus, we say that Allah did not ordain the occurrence of a certain thing, and we do not say that He ordained its non-occurrence.