perlcritic: return undef in list context too

Dependencies

• [2] 6ZB4CIW6 Security: Improve checking of build products
• [3] HTL6HIBM machine-status: Read /etc/nix.machines instead of using the BuildMachines table
• [4] PMNWRTGJ Add multiple output support
• [*] 2GK5DOU7 * Downloading closures.

Change contents

• replacement in src/lib/Hydra/Helper/Nix.pm at line 399
  [2.1431]→[2.1431:1486](∅→∅)
  − return if length($cur) <= length($prefix);
  [2.1431]

  [2.1486]
  + return undef if length($cur) <= length($prefix);