Automatic merge from submit-queue (batch tested with PRs 54800, 53898, 54812, 54921, 53558). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

Allow override of cluster level (default, whitelist) tolerations by namespace level empty (default, whitelist) tolerations.

Currently In PodTolerationRestriction admission plugin, if namespace level default and whitelist of tolerations are nil or empty, they do not override cluster level default and whitelist tolerations. 

This PR fixes the plugin to not override cluster level tolerations only when namespace level toleration are nil. IOW, if namespace level toleration are empty, they override cluster level tolerations. To be more clear, if following annotations are set to empty, they override cluster level tolerations.
 
``` 
scheduler.alpha.kubernetes.io/defaultTolerations : ""
scheduler.alpha.kubernetes.io/tolerationsWhitelist: ""
```

This behavior is inline with PodNodeSelector admission plugin too.

@sjenning @derekwaynecarr 

**Release Note**:

```release-note
In PodTolerationRestriction admisson plugin, if namespace level tolerations are empty, now they override cluster level tolerations. 
```