[SPARK-2093] [SQL] NullPropagation should use exact type value.

`NullPropagation` should use exact type value when transform `Count` or `Sum`.
I agree with that.
Even if we never change/specialize the return type, `SimplifyCasts` can remove the extra `Cast`.

And I'd like to change the `Literal` value for `Sum` to `0L` to use `SimplifyCasts` for the case of the sum of integral values.