tz_localize does not handle NaT
`DatetimeIndex` correctly handles `NaT`:

```
In [2]: pd.DatetimeIndex([pd.NaT])
Out[2]: 
<class 'pandas.tseries.index.DatetimeIndex'>
[NaT]
Length: 1, Freq: None, Timezone: None
```

But calling `tz_localize` leads to garbage:

```
In [1]: pd.DatetimeIndex([pd.NaT]).tz_localize('US/Pacific')
Out[1]: 
<class 'pandas.tseries.index.DatetimeIndex'>
[2262-04-10 00:12:43.145224192]
Length: 1, Freq: None, Timezone: US/Pacific
```
