NPE in canCompleteSwap if path doesn't exists

### Describe the bug
Currently when we call [baseAccessor.getChildNames](https://github.com/apache/helix/blob/62f0e9c814b7543cc26367f1c3841ca0d5db7ae5/helix-core/src/main/java/org/apache/helix/manager/zk/ZKHelixAdmin.java#L552-L554) and if the path doesnt exists, swapOutResources is null, and we [iterate](https://github.com/apache/helix/blob/62f0e9c814b7543cc26367f1c3841ca0d5db7ae5/helix-core/src/main/java/org/apache/helix/manager/zk/ZKHelixAdmin.java#L555) over this which results in NPE

```
"error" : "Cannot invoke \"java.util.List.iterator()\" because \"swapOutResources\" is null"
}
```

### To Reproduce
Hit canCompleteSwap Rest and manually corrupt the path.

### Expected behavior
NPEs should be handled, it should return true in this case as well.

### Additional context
Add any other context about the problem here.