nix/search: no error for empty search results if json is enabled

[Mic92]

• result list will be always empty if --json is passed
• for scripts an empty search result is not really an error,
  we rather want to distinguish between evaluation errors and empty results

[Mic92]

Snippet for quick testing:

{}: {
  nix.package = pkgs.nix.overrideAttrs (old: {
    patches = [
      (pkgs.fetchpatch {
        url = "https://github.com/Mic92/nix-1/commit/e60dc42caa659985aa689958c656f26dc51eae56.patch";
        sha256 = "19cg2q34sir0q83ni6jqvxlglqsz1xld57gizqzvs8g5m5ckb806";
      })
    ];
  });
}

Before:

[nix-shell:~/git/nixpkgs]$ nix search --json > /dev/null
warning: using cached results; pass '-u' to update the cache
error: no results for the given search term(s)!

After

$ nix search --json >/dev/null
warning: using cached results; pass '-u' to update the cache

[edolstra]

Note that it's not really worth fixing bugs in the nix search cache because it has been completely rewritten on the flakes branch.

[domenkozar]

@edolstra sorry for that, do you want to revert?

[Mic92]

No need to revert. The bug is present in both branches. I fixed the flakes branch in #3572

[edolstra]

Ah sorry, I was confused with the JSON cache in the old nix search. I forgot it also has a --json flag for output :-)