It is common for database integers to be represented as int32
internally. Although NullInt64 is already defined,
this should remove some type casts and make working with those eaiser.

For #31231

Change-Id: Ia0c37ecef035fee0734c1d1fb6f58aef6905cf5e
Reviewed-on: https://go-review.googlesource.com/c/go/+/174178
Run-TryBot: Daniel Theophanes <kardianos@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>