optional_args not working with Hiera

When I add an optional_args parameter via Hiera:

```
mysql::server::backup::optional_args: 
  - '--quick'
```

it should add that to the mysqlbackup.sh script, but doesn't. If I change it to a non-array just to intentionally make it invalid, it fails with a puppet error.

I'm using the latest puppetlabs-mysql 16.2.0 at the time of writing.