math/big: produce valid JSON in Int.MarshalJSON when nil

Fixes #50940.

Change-Id: Ie2a0c4505ca9d7e448017d9d00a020a6b3996be3
GitHub-Last-Rev: afd8c6b5598f43de25831c700b8d76cd97571426
GitHub-Pull-Request: golang/go#50941
Reviewed-on: https://go-review.googlesource.com/c/go/+/381963


Trust: Cherry Mui <cherryyz@google.com>
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
Trust: Emmanuel Odeke <emmanuel@orijtech.com>
Run-TryBot: Emmanuel Odeke <emmanuel@orijtech.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Trust: Daniel Martí <mvdan@mvdan.cc>