When they talk about the beginning of the universe, are Theists and physicists talking about an empty universe or no universe at all?
It is fairly straightforward to imagine an empty Universe - an infinite set of points represented by some set of abstract coordinates with no ``objects'' located at any of the points. Hmmmm, isn't that what mathematics is all about for the first umpty years one studies it?
Note well that (empty or not) we can imagine putting things at those coordinates, using those coordinates to label the things and help to sort them into sets (including disjoint ''identity sets''), just as we can imagine putting things ``into'' the empty set (via the Union operation) and creating a non-empty set.
There is, however, a deeper notion of ''emptiness'', that of ``non-being''. The notion of no box at all.