This matches NullBool, NullFloat64, and NullInt64.

Fixes #30305

Change-Id: I79bfcf04a3d43b965d2a3159b0ac22f3e8084a53
Reviewed-on: https://go-review.googlesource.com/c/go/+/170699
Run-TryBot: Daniel Theophanes <kardianos@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>