math/big: big.Int.MarshalJSON produces invalid json for nil

[Snaipe]

What version of Go are you using (go version)?

$ go version
go version go1.17.6 linux/amd64

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/snaipe/.local/var/cache/go-build"
GOENV="/home/snaipe/.local/etc/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/snaipe/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/snaipe/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/lib/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/lib/go/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="go1.17.6"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/home/snaipe/b5/src/snai.pe/boa/go.mod"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build3656356402=/tmp/go-build -gno-record-gcc-switches"

What did you do?

https://play.golang.com/p/Zz6ueutEwo7

What did you expect to see?

MarshalJSON should return the JSON null value.

What did you see instead?

MarshalJSON instead returns <nil> with no error, which is invalid JSON. At the very least, if MarshalJSON should not return null for nil *big.Int, an error should be returned rather than producing invalid JSON.

[gopherbot]

Change https://golang.org/cl/381963 mentions this issue: math: fix big.Int MarshalJSON when x is nil

[nightlyone]

Should the big.Int.Unmarshal also parse this somehow to keep it symmetrical?

[Snaipe]

big.Int.UnmarshalJSON already parses null so this would make things more consistent, as you pointed out.