libfetchers: handle nonexistent refs in GitLab repos more gracefully

Before:

$ nix flake lock --override-input nixpkgs gitlab:simple-nixos-mailserver/nixos-mailserver/nonexistent
fetching git input 'git+file:///home/linus/projects/lix'
fetching gitlab input 'gitlab:simple-nixos-mailserver/nixos-mailserver/nonexistent'
error: [json.exception.type_error.302] type must be string, but is null

After:

/tmp/inst/bin/nix flake lock --override-input nixpkgs gitlab:simple-nixos-mailserver/nixos-mailserver/nonexistent

warning: unknown experimental feature 'repl-flake'
error:
       … while updating the lock file of flake 'git+file:///home/joerg/git/nix?ref=refs/heads/master&rev=62693c2c37c8edd92f95114eb1387b461fc671df'

       … while updating the flake input 'nixpkgs'

       … while fetching the input 'gitlab:simple-nixos-mailserver/nixos-mailserver/nonexistent'

       error: No commits returned by GitLab API -- does the git ref really exist?

Original source: https://git.lix.systems/lix-project/lix/commit/3df013597d7a2b5e400839e6625c05bd47de4dca

src/libfetchers/github.cc

@@ -433,9 +433,15 @@ struct GitLabInputScheme : GitArchiveInputScheme
                 store->toRealPath(
                     downloadFile(store, url, "source", headers).storePath)));
 
-        return RefInfo {
-            .rev = Hash::parseAny(std::string(json[0]["id"]), HashAlgorithm::SHA1)
-        };
+        if (json.is_array() && json.size() == 1 && json[0]["id"] != nullptr) {
+          return RefInfo {
+              .rev = Hash::parseAny(std::string(json[0]["id"]), HashAlgorithm::SHA1)
+          };
+        } if (json.is_array() && json.size() == 0) {
+            throw Error("No commits returned by GitLab API -- does the git ref really exist?");
+        } else {
+            throw Error("Unexpected response received from GitLab: %s", json);
+        }
     }
 
     DownloadUrl getDownloadUrl(const Input & input) const override