Do you notice that the opamp circuits shown have negative feedback so that they have a reasonably low voltage gain? Then the offset is amplified only by the fairly low gain and the adjustment is easy.
But a comparator has no negative feedback so it amplifies the offset voltage 200,000 times (LM339 and LM393) and it will be impossible to adjust an offset voltage null.
Why do you want to make an offset voltage null? The LM339 and LM393 comparators have a typical offset voltage of only 1mV or 2mV max.