Automatic merge from submit-queue (batch tested with PRs 50953, 51082)

Fix mergekey of initializers; Repair invalid update of initializers

Fix https://github.com/kubernetes/kubernetes/issues/51131

The PR did two things to make parallel patching `metadata.initializers.pending` possible:
* Add mergekey to initializers.pending
* Let the initializer admission plugin set the `metadata.intializers` to nil if an update makes the `pending` and the `result` both nil, instead of returning a validation error. Otherwise if multiple initializer controllers sending the patch removing themselves from `pending` at the same time, one of them will get a validation error.


```release-note
The patch to remove the last initializer from metadata.initializer.pending will result in metadata.initializer to be set to nil (assuming metadata.initializer.result is also nil), instead of resulting in an validation error.
```