Concatenating rows with Int64 datatype coerces to object
- [x] closes #24768
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`


Issue was fixed in the past.

The test itself looks good just not sure on location - @jreback should this go in pandas/tests/extension/test_integer.py::TestReshaping or is that intended for something else? I added it there, because directly above is a similar test. If we move the new test, we should move that one too. nah this looks like a good location. thanks @phofl 