Fixed -e "" on ansible-playbook.

Currently, *ansible-playbook* fails when empty extra vars are passed (-e "").

```
Traceback (most recent call last):
  File "/home/alan/workspace/ansible/bin/ansible-playbook", line 268, in <module>
    sys.exit(main(sys.argv[1:]))
  File "/home/alan/workspace/ansible/bin/ansible-playbook", line 106, in main
    elif extra_vars_opt[0] in '[{':
IndexError: string index out of range
```

The case of an empty string is not being accounted for. This commit fixes it.