This argument is as obnoxious as ever. Everything hinges on the idea that you can't prove a negative, but you can. It's easy. For example, I can prove that there's no red pen on my desk, because if there was, it would be visible right now. The lack of evidence is itself evidence if we can show that there otherwise would be evidence here.
Similarly, if there was widespread voter fraud, what would that mean? There would have to be tons of votes that were cast under false names, or something to that effect, right? That sort of thing, if it's widespread, would be easy for an investigation to find. If it's there, it should be visible, and if it isn't visible when it should be, that's evidence that it's just not there, like the red pen.
The proper response to the "There's no evidence that there's not" argument isn't "You can't prove a negative". The proper response is "Yes there is!".
And if you think all of this is incredibly obvious and shouldn't need to be explained, then congratulations. You are smarter than Mark Meadows.