BUG: union_categorical with Series and cat idx
- [x] closes #14173
- [ ] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry (not needed)

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/14199?src=pr) is 85.24% (diff: 100%)

> Merging [#14199](https://codecov.io/gh/pydata/pandas/pull/14199?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will increase coverage by **<.01%**

``` diff
@@             master     #14199   diff @@
==========================================
  Files           140        140          
  Lines         50556      50563     +7   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43095      43102     +7   
  Misses         7461       7461          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [289cd6d...b04ea4d](https://codecov.io/gh/pydata/pandas/compare/289cd6d0df66b812921ff4c5cbade937b875406d...b04ea4d162d98a6ded508abc7a1861ff64057e91?src=pr)
 thanks!
