Directory diff can create an empty directory.

  $ cat > dune-project <<'EOF'
  > (lang dune 3.23)
  > (using directory-targets 0.1)
  > EOF

  $ mkdir expected
  $ cat > dune <<'EOF'
  > (rule
  >  (targets (dir actual))
  >  (action (system "mkdir -p actual/empty")))
  > 
  > (rule
  >  (alias runtest)
  >  (action (diff expected actual)))
  > EOF

  $ dune runtest
  File "dune", lines 5-7, characters 0-56:
  5 | (rule
  6 |  (alias runtest)
  7 |  (action (diff expected actual)))
  Error: Directory expected/empty should be created
  [1]

  $ dune promote

  $ test -d expected/empty && echo created
  created
