--format=junit doesn't work without --out also passed

[dkarlovi]

 ̇$ vendor/bin/behat --format=junit

Deprecated: is_file(): Passing null to parameter #1 ($filename) of type string is deprecated in /app/vendor/behat/behat/src/Behat/Testwork/Output/Printer/Factory/FilesystemOutputFactory.php on line 47

Deprecated: is_dir(): Passing null to parameter #1 ($filename) of type string is deprecated in /app/vendor/behat/behat/src/Behat/Testwork/Output/Printer/Factory/FilesystemOutputFactory.php on line 52

Deprecated: mkdir(): Passing null to parameter #1 ($directory) of type string is deprecated in /app/vendor/behat/behat/src/Behat/Testwork/Output/Printer/Factory/FilesystemOutputFactory.php on line 53

Warning: mkdir(): Invalid path in /app/vendor/behat/behat/src/Behat/Testwork/Output/Printer/Factory/FilesystemOutputFactory.php on line 53

Warning: fopen(/file.xml): Failed to open stream: Permission denied in /app/vendor/behat/behat/src/Behat/Testwork/Output/Printer/Factory/FilesystemOutputFactory.php on line 63

                                                                
  The StreamOutput class needs a stream as its first argument.  

It requires you to also pass --out, but that's not clear from the error message at all.

Options from discussion in Symfony Slack:

1. default to CWD
2. clear exception that --out is now required