Fix promotion of directory targets without files in them #29283

Job	Run time
Coverage (5.1.x)	9m 27s
Build microbenchmarks	2m 32s
Build (macos-latest, 5.1.x)	15m 29s
Coq 8.16.1	7m 28s
Build (ubuntu-latest, 5.1.x)	8m 49s
Build (ocaml-variants.5.3.0+trunk, ubuntu-latest, true)	1m 42s
Create local opam switch (macos-latest, 5)	6m 44s
Documentation	1m 0s
Build (4.14.x, ubuntu-latest, true)	1m 26s
Create local opam switch (macos-latest, 4.14)	6m 32s
Format	1m 8s
Build (4.14.x, macos-latest, true)	3m 3s
Create local opam switch (ubuntu-latest, 5)	5m 2s
Build (5.1.x, macos-13, true)	5m 41s
Create local opam switch (ubuntu-latest, 4.14)	4m 47s
Build monorepo benchmark docker image	3m 59s
Build (ocaml-base-compiler.4.14.2,system-msvc, windows-latest)	14m 25s
Nix (macos-latest)	2m 5s
Build (ocaml-base-compiler.5.1.1,system-mingw, windows-latest)	8m 23s
Nix (ubuntu-latest)	54s
Build (4.08.x, ubuntu-latest, true)	1m 27s
Wasm_of_ocaml	10m 47s
Build (4.04.x, ubuntu-latest, true, true)	4m 34s
Build (4.02.x, ubuntu-latest, true)	4m 7s
	2h 11m 31s

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