PERF: Faster Series construction with no data and DatetimeIndex.
closes pydata/pandas#11433

Code. taken from @jreback comment on pydata/pandas#11433

asv benchmark:

```
    before     after       ratio
  [6c4d2c7d] [6439bd94]
-     1.68s   364.81μs      0.00  series_methods.series_constructor_no_data_datetime_index.time_series_constructor_no_data_datetime_index
SOME BENCHMARKS HAVE CHANGED SIGNIFICANTLY.
```

lgtm. add a whatsnew note (perf section).
of course if you can make the code I did better / simplier, pls do so.
 ping when note added, ready, and green.
 done, done, and done ;)
 minor comments, ping when green.
 changes made, waiting for Travis again
 green. ping @jreback 
 thanks!
