PackageSelection fails when a package json is an empty array.

```bash
composer_1 | Downloading https://packages.drupal.org/8/drupal/rules_ui%244f53cda18c2baa0c0354bb5f9a3ecbe5ed12ab4d8e11ba873c2f11161202b945.json
composer_1 | Writing /composer/cache/repo/https---packages.drupal.org-8/provider-drupal$rules-ui.json into cache
composer_1 |
composer_1 | In ComposerRepository.php line 376:
composer_1 |
composer_1 | [ErrorException]
composer_1 | Undefined index: packages
composer_1 |
composer_1 |
composer_1 | Exception trace:
composer_1 | at /satis/vendor/composer/composer/src/Composer/Repository/ComposerRepository.php:376
composer_1 | Composer\Util\ErrorHandler::handle() at /satis/vendor/composer/composer/src/Composer/Repository/ComposerRepository.php:376
composer_1 | Composer\Repository\ComposerRepository->whatProvides() at /satis/vendor/composer/composer/src/Composer/DependencyResolver/Pool.php:204
composer_1 | Composer\DependencyResolver\Pool->computeWhatProvides() at /satis/vendor/composer/composer/src/Composer/DependencyResolver/Pool.php:193
composer_1 | Composer\DependencyResolver\Pool->whatProvides() at /satis/src/PackageSelection/PackageSelection.php:685
composer_1 | Composer\Satis\PackageSelection\PackageSelection->selectLinks() at /satis/src/PackageSelection/PackageSelection.php:188
composer_1 | Composer\Satis\PackageSelection\PackageSelection->select() at /satis/src/Console/Command/BuildCommand.php:184
composer_1 | Composer\Satis\Console\Command\BuildCommand->execute() at /satis/vendor/symfony/console/Command/Command.php:255
composer_1 | Symfony\Component\Console\Command\Command->run() at /satis/vendor/symfony/console/Application.php:934
composer_1 | Symfony\Component\Console\Application->doRunCommand() at /satis/vendor/symfony/console/Application.php:273
composer_1 | Symfony\Component\Console\Application->doRun() at /satis/src/Console/Application.php:49
composer_1 | Composer\Satis\Console\Application->doRun() at /satis/vendor/symfony/console/Application.php:149
composer_1 | Symfony\Component\Console\Application->run() at /satis/bin/satis:26
composer_1 |
composer_1 | build [--repository-url [REPOSITORY-URL]] [--repository-strict] [--no-html-output] [--skip-errors] [--stats] [--] [ [ [...]]]
composer_1 |
drupal-composer-mirror_composer_1 exited with code 1
```

`/composer/cache/repo/https---packages.drupal.org-8/provider-drupal$rules-ui.json` file's content is an empty array.