Variables lookup in a template should handle properly the undefined case

Hi,

If I create a role that is only doing this action :
```ansible
  - authorized_key: user=deploy key="{{ lookup('file', item) }}"
    with_fileglob:
    - ../keys/*
```

I end up with the following error: ```ERROR: Unexpected error in during lookup: 'item' is undefined```.

With the provided patch, Ansible is successfully executing the role.
I'm not familiar with the code base, let me know if there is a better solution to this issue.

Thanks