BUG: fix fill value for gouped sum in case of unobserved categories for string dtype (empty string instead of 0)
I ran into one more case of the sum of empty / all-NaN to use "0" instead of empty string (https://github.com/pandas-dev/pandas/issues/60229), specifically when effectively introducing empty groups with categorical data with observed=False.

Follow-up on https://github.com/pandas-dev/pandas/pull/60936 

The passing through of `is_string` through several layers is a bit annoying, but effectively is needed to for now only changes this for string dtype, and not for object dtype in general (which in the other PR related to this, we did for now)
Owee, I'm MrMeeseeks, Look at me.

There seem to be a conflict, please backport manually. Here are approximate instructions:

1. Checkout backport branch and update it.

```
git checkout 2.3.x
git pull
```

2. Cherry pick the first parent branch of the this PR on top of the older branch:
```
git cherry-pick -x -m1 27928edc61f5b01e933036a99549636425e5a557
```

3. You will likely have some merge/cherry-pick conflict here, fix them and commit:

```
git commit -am 'Backport PR #61909: BUG: fix fill value for gouped sum in case of unobserved categories for string dtype (empty string instead of 0)'
```

4. Push to a named branch:

```
git push YOURFORK 2.3.x:auto-backport-of-pr-61909-on-2.3.x
```

5. Create a PR against branch 2.3.x, I would have named this PR:

> "Backport PR #61909 on branch 2.3.x (BUG: fix fill value for gouped sum in case of unobserved categories for string dtype (empty string instead of 0))"

And apply the correct labels and milestones.

Congratulations — you did some good work! Hopefully your backport PR will be tested by the continuous integration and merged soon!

Remember to remove the `Still Needs Manual Backport` label once the PR gets merged.

If these instructions are inaccurate, feel free to [suggest an improvement](https://github.com/MeeseeksBox/MeeseeksDev).
                 Manual backport -> https://github.com/pandas-dev/pandas/pull/61963