Add default platform value to #fetch_local_hash

[SkyWriter]

Not having this pulls in the wrong target value into gemset.nix for gems like so:

  coderay = {
    groups = ["default"];
    platforms = [];
    source = {
      remotes = ["https://rubygems.org"];
      sha256 = "0jvxqxzply1lwp7ysn94zjhh57vc14mcshw1ygw14ib8lhc00lyw";
      target = null;
      type = "gem";
    };
    targets = [];
    version = "1.1.3";
  };

Whereas it should be "ruby" instead:

  coderay = {
    groups = ["default"];
    platforms = [];
    source = {
      remotes = ["https://rubygems.org"];
      sha256 = "0jvxqxzply1lwp7ysn94zjhh57vc14mcshw1ygw14ib8lhc00lyw";
      target = "ruby";
      type = "gem";
    };
    targets = [];
    version = "1.1.3";
  };

Which results in a weird Nix shell error:

error:
       … while calling the 'derivationStrict' builtin

         at /builtin/derivation.nix:9:12: (source not available)

       … while evaluating derivation 'nix-shell'
         whose name attribute is located at /nix/store/6nfpbajyzwxq6wrhn20l13rhckhv26rc-source/pkgs/stdenv/generic/make-derivation.nix:348:7

       … while evaluating attribute 'buildInputs' of derivation 'nix-shell'

         at /nix/store/6nfpbajyzwxq6wrhn20l13rhckhv26rc-source/pkgs/stdenv/generic/make-derivation.nix:395:7:

          394|       depsHostHost                = elemAt (elemAt dependencies 1) 0;
          395|       buildInputs                 = elemAt (elemAt dependencies 1) 1;
             |       ^
          396|       depsTargetTarget            = elemAt (elemAt dependencies 2) 0;

       (stack trace truncated; use '--show-trace' to show the full trace)

       error: cannot coerce null to a string

One can reproduce it by:

$ nix flake init -t github:inscapist/ruby-nix/main
$ nix develop
$ echo 'source "https://rubygems.org"' > Gemfile
$ echo 'gem "pry"' >> Gemfile
$ bundle-lock
$ bundix
$ exit
$ nix develop

It may fail to reproduce on the first take since it seems to be caused by the branch fetching gems from the local store.

[SkyWriter]

@inscapist , thank you for the wonderful work on this!

I am wondering if I am going in a right direction, and what else needs to be done for this PR to land?

[SkyWriter]

Ultimately, the issue seems to stem from this value remaining nil as never assigned for the gems that don't have platform dependent versions. This nil ended up being returned on https://github.com/inscapist/bundix/pull/12/files#diff-6c1d40dc95e017380bc6a0cd183b22f63fef5ac09356ae7ba8a288009a6355c7R128 .

I've amended it by pre-assigning the default value.

[inscapist]

sorry @SkyWriter I was swarmed in work! let me review this

[inscapist]

lgtm

[SkyWriter]

No worries! Thank you for your hard work!