TST: Testing that no warnings are emitted and that inplace fillna produces the correct result (GH48480)
- [x] closes #48480
- [x] [Tests added and passed]
- [x] All [code checks passed]

In GH48480 it was stated that using fillna in-place on a dataframe with Nones emits a future warning. The issue was already resolved as also stated in the issue, so I included a test which tests on an example that no warning is emitted and that the result is correct.
thx @RaphSku 