DESPITE the fact that there's a `Background()` API that we
  could just call like:

  ```c++
      TabViewItem().Background(deselectedTabBrush);
  ```

  We actually can't, because it will make the part of the tab that
  doesn't contain the text totally transparent to hit tests. So we
  actually _do_ still need to set `TabViewItemHeaderBackground` manually.

  * Regressed in #11240
  * Root cause up in microsoft/microsoft-ui-xaml#3769
  * [x] closes #11294

DESPITE the fact that there's a `Background()` API that we
could just call like:

```c++
  TabViewItem().Background(deselectedTabBrush);
```

We actually can't, because it will make the part of the tab that
doesn't contain the text totally transparent to hit tests. So we
actually _do_ still need to set `TabViewItemHeaderBackground` manually.

* Regressed in #11240
* Root cause up in microsoft/microsoft-ui-xaml#3769
* [x] closes #11294

I've had a hard time with the tab colors this week.

Turns out that setting the background to nullptr will make the tabviewitem invisible to hit tests. `Transparent`, on the other hand, is totally valid, and the expected default. 

Tabs as of this commit:

![tab-color-fix-3](https://user-images.githubusercontent.com/18356694/135915272-ff90b28b-f260-493e-bf0b-3450b4702dce.gif)

## PR Checklist
* [x] Closes #11382
* [x] I work here

This low-key reverts a bit of #11369, which fixed #11294, which regressed in #11240