opam install fails when having pinned a package with an empty opam file inside its archive

[kit-ty-kate]

When calling opam pin add <pkg>, opam install <pkg>, opam remove <pkg> then opam install --with-test <pkg> with <pkg> being a package with an empty opam file inside its archive opam will fail with:

The following actions will be performed:
  - install unison 2.51.3*
'opam install --verbose --yes unison.2.51.3 --with-test' failed.
Fatal error:
In /home/opam/.opam/4.10/.opam-switch/sources/unison/unison.opam:
unsupported or missing file format version