nix search --json: Don’t fail for empty result set

[pmiddend]

I'm using nix search --json to generate a list of all the packages available for installation. I noticed that I cannot test this command for failure, because...it always fails.

Specifically, it always outputs "no results for the given search term(s)", even after outputting a huge list of packages, and it always results in an exit code of 1.

Is this intentional? I see two cases to handle here:

1. The user doesn't provide a search string, so nix search or nix search --json. This is valid and even an example in the manpage. In this case, I think you never want to fail.
2. The user provides a search string, and nothing is found. In this case, failing (and outputting a message on stderr) is good behavior when --json isn't provided. If it is, I think it's a matter of preference. I personally wouldn't let it fail. The script you're writing can process the JSON and check for emptiness.

Regarding the code: Since results is apparently not filled in json mode, testing it for emptiness doesn't make much sense. This PR only tests this in plain-text mode as a first fix to the problem.

[evils]

duplicate of #2266 and #2657?

[pmiddend]

Apparently. Sorry for that. Curious though, why does it take so long to merge this?

[Mic92]

Fixed in #3572 and #3570