localize_tso unnecessary calls?
Related: #17876, #17734 

In `tslib._localize_tso` there are 4 of blocks of the form:

```
        if obj.value != NPY_NAT:
            pandas_datetime_to_datetimestruct(obj.value + delta,
                                              PANDAS_FR_ns, &obj.dts)
        else:
            pandas_datetime_to_datetimestruct(obj.value,
                                              PANDAS_FR_ns, &obj.dts)

```

But in the case where `obj.value == NPY_NAT`, can't we skip the call to `pandas_datetime_to_datetimestruct` because we're going to end up with `NaT` anyway?