BUG: Preserve sparse dtype when reindexing
- [ ] closes #26123.
- [ ] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Hi, I've been taking a look at this but I need to verify some of the surrounding functionality so that I prevent introducing more breakages.

In this test, https://github.com/alex-hutton/pandas/blob/preserve-sparse-dtype-26123/pandas/tests/sparse/test_combine_concat.py#L99-L110 , my interpretation is that its purpose is to verify that if two `SparseSeries` are concatenated, a `fill_value` set on either of those `SparseSeries` should be ignored in favour of the default `fill_value` of `NaN`.

Is my interpretation correct, and is this test correct? It seems odd that the the `fill_value` of the `SparseSeries` should be disregarded, but I am probably missing something.