#2323 added `EmptyBoxes` to `DistributedBoxCollection` as well as `BoxCollection`

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@18248 b5f69653-2bf5-0310-8201-b0dafb8e1405
git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@18256 b5f69653-2bf5-0310-8201-b0dafb8e1405