UI error in the alloc details page when the allocation is rescheduled

### Nomad version
`v1.2.4`

### Operating system and Environment details

### Issue
When accessing an allocation with a failed task, the allocation details page throw this error:

```
Uncaught TypeError: task is undefined
getTaskSortPrefix http://localhost:4200/ui/assets/nomad-ui.js:8123
_dec3 http://localhost:4200/ui/assets/nomad-ui.js:8042
customSort http://localhost:4200/ui/assets/vendor.js:46525
customSort http://localhost:4200/ui/assets/vendor.js:46525
arrayMacro http://localhost:4200/ui/assets/vendor.js:45184
get http://localhost:4200/ui/assets/vendor.js:20890
untrack http://localhost:4200/ui/assets/vendor.js:61061
get http://localhost:4200/ui/assets/vendor.js:20889
...
```
Full API JSON result and browser error stack: https://gist.github.com/lgfa29/9435ccaaf0f810ae56776fe7fa6c016a


### Reproduction steps

The job I used was a mess, but I will try to create a simple job that reproduces this

Start at the allocation details page and reproduce the error with an allocation containing a failed task that has been rescheduled. Compare the full API JSON and browser stack trace in the linked gist with the generated nomad-ui.js locations; done means the page renders without the “task is undefined” error for that allocation.

Written by the indexing model from the issue text.