BUG: Fix min_count issue for groupby.sum
- [x] closes #32861
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

@dsaxton what's the status here? > @dsaxton what's the status here?

It's working / green, but I think @jreback may want it refactored (I haven't had much success in deleting what looks like redundant code without causing tests to fail) The 'regression' for Int64 could be considered a bug since Int64 is considered experimental. 

I've not looked in detail yet, but the 'regression' was caused by #31359 see https://github.com/pandas-dev/pandas/issues/32861#issuecomment-605453895 so would to check that the other EAs which aren't experimental aren't affected by this issue.


> but I think @jreback may want it refactored

**if** this issue **does** affect other EAs, then this PR is a candidate for backport, so would rather keep the changes here minimal. > **if** this issue **does** affect other EAs, then this PR is a candidate for backport, so would rather keep the changes here minimal.

It doesn't affect others that I'm aware of (there's a similar but as far as I can tell unrelated problem with boolean data type which I created a separate issue for) @jreback @simonjayhawkins Is is possible we can merge this as is? I have a fix for a separate bug that depends on this. > @jreback @simonjayhawkins Is is possible we can merge this as is? I have a fix for a separate bug that depends on this.

sure is this rebase on master? > sure is this rebase on master?

I think Simon merged this morning > > **if** this issue **does** affect other EAs, then this PR is a candidate for backport, so would rather keep the changes here minimal.
> 
> It doesn't affect others that I'm aware of (there's a similar but as far as I can tell unrelated problem with boolean data type which I created a separate issue for)

there is a problem also with timedelta, see https://github.com/pandas-dev/pandas/issues/34051#issuecomment-626688541, but this also occurs on 0.25.3

so won't backport for now.



