Without the change metadata evaluation fails on package like
`zammad.src` where no fields are defined in `.nix `files:
src = fetchFromGitHub (lib.importJSON ./source.json);
There evaluation fails as:
$ nix-instantiate --strict --eval --expr 'with import ./. {}; zammad.src.meta'
error:
23| # to indicate where derivation originates, similar to make-derivation.nix's mkDerivation
24| position = "${position.file}:${toString position.line}";
| ^
25| };
error: value is null while a set was expected
After the change evaluation succeeds as:
$ nix-instantiate --strict --eval --expr 'with import ./. {}; zammad.src.meta'
{ homepage = "https://github.com/zammad/zammad"; }
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