This PR was merged into the 3.8.x-dev branch.

Discussion
----------

I added "return 0" to fix this error message:

```
$ vendor/bin/behat --debug

behat version 3.7.0

--- configuration
    environment variable (BEHAT_PARAMS): none
    configuration file: /home/jawira/PhpstormProjects/xxxxxxxx/behat.yml

--- extensions
    extensions loaded: api_extension

PHP Fatal error:  Uncaught TypeError: Return value of "Behat\Testwork\Cli\DebugCommand::execute()" must be of the type int, NULL returned. in /home/jawira/PhpstormProjects/xxxxxxxx/vendor/symfony/console/Command/Command.php:258
Stack trace:
#0 /home/jawira/PhpstormProjects/xxxxxxxx/vendor/symfony/console/Application.php(912): Symfony\Component\Console\Command\Command->run()
#1 /home/jawira/PhpstormProjects/xxxxxxxx/vendor/symfony/console/Application.php(264): Symfony\Component\Console\Application->doRunCommand()
#2 /home/jawira/PhpstormProjects/xxxxxxxx/vendor/behat/behat/src/Behat/Testwork/Cli/Application.php(124): Symfony\Component\Console\Application->doRun()
#3 /home/jawira/PhpstormProjects/xxxxxxxx/vendor/symfony/console/Application.php(140): Behat\Testwork\Cli\Application->doRun()
#4 /home/jawira/PhpstormProjects/xxxxxxxx/vendor/behat/behat/bin/behat(34): Symfony\Component\Console\Application->run()
#5 {main}
  thrown in /home/jawira/PhpstormProjects/xxxxxxxx/vendor/symfony/console/Command/Command.php on line 258
```

Commits
-------

48a3636 Add "return 0" in DebugCommand.php
902b3c6 Add debug test