Index.map followups
xref #15081, a couple of issues

- [ ] ``IntervalIndex.map`` is xfailed
- [x] parametrize [indexex/common.py/test_map](https://github.com/pandas-dev/pandas/blob/master/pandas/tests/indexes/common.py#L1009) similar to how ``indexex/datetimelike.py/test_map`` is done (IOW on dict / Series) (#18491)
- [x] resolve datetimelike with empty map [2] should be all ``NaT`` for the datetimelikes. (#18491)

```
In [2]: pd.date_range('20130101', periods=3).map({})
Out[2]: Float64Index([nan, nan, nan], dtype='float64')

In [3]: pd.Index([1,2,3]).map({})
Out[3]: Float64Index([nan, nan, nan], dtype='float64')
```
