TST: move Series.map tests to tests/series/methods/test_map.py
Currently the tests for `Series.map` are mixed together with the tests for `Series.apply`. Keeping them separate makes it easier to find tests for one method and I've moved the map ones to tests/series/methods/test_map.py.

Also fixed a bug that I found in this proces:

```python
>>> import pandas as pd
>>> ser = pd.Series([], dtype=int)
>>> ser.map(lambda: x + 1).dtype
object  # main
int64  # this PR
```

Thanks @topper-123 