Magento 2 recipe: artifact_excludes_file should default to null, and support a null value

[joeshelton-wagento]

When running the artifact:build command of the Magento 2 recipe, I receive the following error:

[localhost] Deployer\Exception\GracefulShutdownException in magento2.php on line 461:
[localhost]
[localhost] No artifact excludes file provided, provide one at artifacts/excludes or change location
[localhost]

I don't have anything to exclude and would prefer not to create an empty excludes file. artifact_excludes_file should default to null, and support a null value.