Half-Blood Prince shows us a charm that can refill drinks, neatly called the Refilling Charm. Does this contradict Gamp’s Law of Elemental Transfiguration, which tells us that you can't transfigure food? The line between food and drink is quite blurry. For example, you can drink soup and custard.
Gamp's law states that one cannot create certain 5 objects from nothing, one of these objects being food. However, one can multiply, increase, summon or transfigure certain food he has, as Hermione explains:
"Your mother can't produce food out of thin air," said Hermione. "no one can. Food is the first of the five Principal Exceptions to Gamp's Law of Elemental Transfiguration."
"Oh, speak English, can't you?" Ron said, prying a fish out from between his teeth.
"It's impossible to make good food out of nothing! You can Summon it if you know where it is, you can transform it, you can increase the quantity if you've already got some"
Hermione's own statement of the law has two clauses that could work
Summon it if you know where it is,
Possible in both cases where we see it: Harry might have seen where the original wine bottle was, and certainly at a wedding, they would have champagne.
increase the quantity if you've already got some
All you have to do here is specify that the glass was not left absolutely dry when "empty" and the refilling is increasing the traces of the drink left. Perhaps this is why it is a refilling, not a filling, charm.