Cherry-pick fix for empty map check crash to 29.x#20904

anandolee merged 1 commit intoprotocolbuffers:29.xprotocolbuffers/protobuf:29.xfrom

neilconway:empty-map-check-backport-29.xneilconway/protobuf:empty-map-check-backport-29.xCopy head branch name to clipboard