Backport PR #55619 on branch 2.1.x (BUG: Groupby not keeping string dtype for empty objects)
* BUG: Groupby not keeping string dtype for empty objects

* Fix

Backports PR #55619

Just needed to move a test for this one. Ack, looks like there's a segmentation fault here.
(not sure if it's spurious or not, it happens after the tests finish?)

see https://github.com/pandas-dev/pandas/actions/runs/6654698565/job/18083267496?pr=55705#step:9:467.

@phofl I'm gonna bump this off the milestone then. Yeah no worries  Nevermind about the segfault, it seems to be unrelated to this PR (I saw it happen on the 2.1.x branch too, and I made a comment about it on the release issue).
https://dev.azure.com/conda-forge/feedstock-builds/_build/results?buildId=812339&view=logs&j=385a3385-ae8b-54d1-9abe-3c614f74cb73&t=d0913101-e169-5874-b37d-533167693e73&l=1989.

I'll send a PR to move the release note. Oops, looks like I accidentally hit close on this for some reason.

(I think I meant to hit merge). Merging, 

I need to move the release note now.