expvar: don't crash if map value set to nil

expvar: don't crash if map value set to nil

Fixes #52719

Change-Id: Ib032193d00664090c47ae92e7d59674ec2d0165a
Reviewed-on: https://go-review.googlesource.com/c/go/+/408677
Reviewed-by: Alan Donovan <adonovan@google.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Jonathan Amsterdam <jba@google.com>