Was the trip to the cave really useless?

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Did Dumbledore really weaken himself for nothing? The locket wasnt slytherins.. Did dumbledore know that. or was this one of his mistakes or bad luck or was it all planned? Did dumbledore know that the locket might not be present? He had done a lot of study on voldemort and the horcruxes an should have known that R.A.B (or rather Regulas Black) could have destroyed it.