Fixes assertion failure if outputsToInstall is empty by defaulting to the "out"
output. That is, behavior between the following commands should be consistent:
$ nix build --no-link --json .#nothing-to-install-no-out
error: derivation '/nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-nothing-to-install-no-out.drv' does not have wanted outputs 'out'
$ nix build --no-link --file default.nix --json nothing-to-install-no-out
error: derivation '/nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-nothing-to-install-no-out.drv' does not have wanted outputs 'out'
Real-world example of this issue:
$ nix build --json .#.legacyPackages.aarch64-linux.texlive.pkgs.iwona
error: derivation '/nix/store/dj0h6b0pnlnan5nidnhqa0bmzq4rv6sx-iwona-0.995b.drv' does not have wanted outputs 'out'
$ git rev-parse HEAD
eee33247cf6941daea8398c976bd2dda7962b125
$ nix build --json --file . texlive.pkgs.iwona
nix: src/libstore/outputs-spec.hh:46: nix::OutputsSpec::Names::Names(std::set<std::__cxx11::basic_string<char> >&&): Assertion `!empty()' failed.
Aborted (core dumped)
68090d7ff1