No error is raised when building an empty package not defined in the dune-project file

It looks like somewhere between the merge of https://github.com/ocaml/dune/pull/4867 and the release of dune 3.0.0, the feature lost all its tests in https://github.com/ocaml/dune/pull/5120 and suffers from a regression:

$ cat dune-project
(lang dune 3.0)
$ touch test.opam

$ dune build # With dune >= 3.0.3 no error

$ ~/dune/dune.exe build # With 5c6eb65b149a9254dc2e9bd618df9e198a321705 we get the error
Error: The package test does not have any user defined stanzas attached to it.
If this is intentional, add (allow_empty) to the package definition in the
dune-project file
-> required by _build/default/test.install
-> required by alias all
-> required by alias default

$ echo "(package (name test))" >> dune-project
$ dune build # now dune >= 3.0.3 shows a warning
Error: The package test does not have any user defined stanzas attached to it.
If this is intentional, add (allow_empty) to the package definition in the
dune-project file
-> required by _build/default/test.install
-> required by alias all
-> required by alias default

Start by reproducing the issue with an empty dune-project and test.opam, then compare the behavior around PRs 4867 and 5120. Check how dune build handles the generated package and install target. Done means restoring the intended error or warning behavior and adding regression coverage for both cases.