how does the controller know for sure there is nothing on the runway.
Management of runway occupancy has been the very raison d'etre of an 'ATCO' since the day that more than one aircraft wished to operate from the same runway.
'The Runway' is THE most highly sought-after piece of 'airspace'.
In Air Traffic Control runway occupancy is managed to the highest possible degree because there is no margin for error. Or even DIY landing clearances.
I don't think the controller would allow anything out on the runway when there is an aircraft on short final with no com (or with com if that matters) or I'm I wrong?
Which would also include ANOTHER aircraft on short final with no com! There is always, at least, the risk of an incursion, an emergency runway crossing requirement, a late report of a broken pit-cover etc.
The captain of the aircraft on final without the landing clearance (for whatever reason) CANNOT know the full picture and DOES NOT have complete information. To land would be reckless.
As BOAC has written:
No landing clearance = do not land. Simple as.
Why is this difficult to understand?