When Voldemort placed his horcrux locket in the cave, he used Kreacher to drink the green potion before placing the locket in the goblet.
When Regulus found the cave, he drank the potion himself before Kreacher took the locket.
When Dumbledore and Harry went in the cave, Dumbledore drank the potion. So the potion drinking was essential to get the locket.
But when Voldemort found out Harry has taken the cup from Lestrange's vault he went to the cave alone to check whether locket was safe or not. Remember the potion wasn't transparent as Harry and Dumbledore didn't see the locket when they saw the goblet. They could only see the potion. And potion used to refill as it refilled after Regulus switched the lockets.
So, who drank the potion that time when Voldemort was checking for the safety of the locket?