[yaml:merge] command requires target file to exist before merging

[freelock]

Problem/Motivation

When using drupal yaml:merge target.yml source1.yml source2.ytml, target.yml is supposed to get the merged yaml from the remaining passed filenames.

This works fine if target.yml exists - e.g. if you touch it first.

If target.yml does not exist, Drupal Console throws an exception:

PHP Warning:  file_put_contents(): Filename cannot be empty in phar:///usr/local/bin/drupal/vendor/drupal/console-core/src/Command/Yaml/MergeCommand.php on line 100
PHP Stack trace:
PHP   1. {main}() /usr/local/bin/drupal:0
PHP   2. require() /usr/local/bin/drupal:10
PHP   3. require() phar:///usr/local/bin/drupal/bin/drupal:3
PHP   4. Symfony\Component\Console\Application->run() phar:///usr/local/bin/drupal/bin/drupal.php:112
PHP   5. Drupal\Console\Launcher\Application->doRun() phar:///usr/local/bin/drupal/vendor/symfony/console/Application.php:122
PHP   6. Drupal\Console\Core\Application->doRun() phar:///usr/local/bin/drupal/src/Application.php:53
PHP   7. Symfony\Component\Console\Application->doRun() phar:///usr/local/bin/drupal/vendor/drupal/console-core/src/Application.php:115
PHP   8. Symfony\Component\Console\Application->doRunCommand() phar:///usr/local/bin/drupal/vendor/symfony/console/Application.php:191
PHP   9. Symfony\Component\Console\Command\Command->run() phar:///usr/local/bin/drupal/vendor/symfony/console/Application.php:868
PHP  10. Drupal\Console\Core\Command\Yaml\MergeCommand->execute() phar:///usr/local/bin/drupal/vendor/symfony/console/Command/Command.php:264
PHP  11. file_put_contents() phar:///usr/local/bin/drupal/vendor/drupal/console-core/src/Command/Yaml/MergeCommand.php:100