Not sure how Mueller gets away with no report, with no absolute clearance of Pres. Trump.
That leaves the door open for endless House investigations (I think there are now 6) of all Trump matters.
Ken Starr operated under a law that created special prosecutors. Part of that law require him (and his successor) to file reports on the matter under investigation. That law no longer exists.
Mueller was merely appointed by Sessions, and so has to follow the same rules as all other Federal prosecutors. As a matter of course they don't write long reports about what they think about the things they investigate. They charge crimes when they find them.
Prosecutors do file a short memo with their supervisors explaining why they charged or chose not to charge key individuals. These are never released to the public.
Because they discuss innocent people, or at least those not accused of any crime.
Comey violated this rule with his first Clinton exhonoration, and was criticized by everyone, left and right.
I expect Mueller to do exactly what is required, a short memo to his boss, and nothing else.