check if HYDRA_DBI is defined in stead of empty string comparison

Dependencies

• [2] VKOHKP4M Hydra now uses an optional HYDRA_DBI environment variable which holds the DBI url. If this variable is not set, sqlite is used like before.
• [3] AFTXA575 * $HYDRA_DATA environment variable.
• [4] 2GK5DOU7 * Downloading closures.

Change contents

• replacement in src/lib/Hydra/Helper/Nix.pm at line 69
  [2.32]→[2.32:53](∅→∅)
  − if ($db ne "") {
  [2.32]

  [2.53]
  + if ( defined $db ) {