Bad freq invalidation in DatetimeIndex.where
What's the expected output here?

```python
In [16]: i = pd.date_range('20130101', periods=3, tz='US/Eastern')

In [17]: i2 = pd.Index([pd.NaT, pd.NaT] + i[2:].tolist())

In [18]: i.where(pd.notna(i2), i2)
Out[18]: DatetimeIndex(['NaT', 'NaT', '2013-01-03 00:00:00-05:00'], dtype='datetime64[ns, US/Eastern]', freq='D')
```

The returned DatetimeIndex doesn't pass freq validation.

```python
In [23]: result._eadata._validate_frequency(result, result.freq)
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
~/sandbox/pandas-alt/pandas/core/arrays/datetimelike.py in _validate_frequency(cls, index, freq, **kwargs)
    863                                           periods=len(index), freq=freq,
--> 864                                           **kwargs)
    865             if not np.array_equal(index.asi8, on_freq.asi8):

~/sandbox/pandas-alt/pandas/core/arrays/datetimes.py in _generate_range(cls, start, end, periods, freq, tz, normalize, ambiguous, nonexistent, closed)
    299         if start is NaT or end is NaT:
--> 300             raise ValueError("Neither `start` nor `end` can be NaT")
    301

ValueError: Neither `start` nor `end` can be NaT

During handling of the above exception, another exception occurred:

ValueError                                Traceback (most recent call last)
<ipython-input-23-24fa3f452eb0> in <module>
----> 1 result._eadata._validate_frequency(result, result.freq)

~/sandbox/pandas-alt/pandas/core/arrays/datetimelike.py in _validate_frequency(cls, index, freq, **kwargs)
    877             raise ValueError('Inferred frequency {infer} from passed values '
    878                              'does not conform to passed frequency {passed}'
--> 879                              .format(infer=inferred, passed=freq.freqstr))
    880
    881     # monotonicity/uniqueness properties are called via frequencies.infer_freq,

ValueError: Inferred frequency None from passed values does not conform to passed frequency D

```

Should the freq be None?