fix the 'KeyError' when skipping asynchronous tasks with poll: 0

This is related to #7432.
I ran into this issue just now, so I wrote this patch. Hopefully it'll fix the issue. 

The following playbook will reproduce the exception.
```yaml
---
- hosts: localhost
  tasks:
  - name: test conditional "fire and forget" task (when skipped)
    command: sleep 5
    poll: 0
    async: 10
    when: False
```
It could be replaced with `if not res.get('skipped'):`