* Properly handle user selection of `None` as vars_files
In a playbook, if a user has a playbook like:
```
- hosts: localhost
connection: local
vars_files:
tasks:
- ....
```
Then `vars_files` will be none, and cause a `TypeError` in vars-manager when it
tries to iterate over them. To avoid this, I changed the getter to either send
back the vars files from the user, or an empty list when the user passed
`None`.
* Only replace None with an empty list, not all falsey values
* Catch error when vars_files isn't iterable
* Move whole `for` loop into try/except and catch TypeError
* Line length
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