Fix crash in node when mixing sync/async resolvers (backport of #3706)#3708

IvanGoncharov merged 2 commits intographql:15.x.xgraphql/graphql-js:15.x.xfrom

chrskrchr:ck.unhandledRejection2.15.x.xchrskrchr/graphql-js:ck.unhandledRejection2.15.x.xCopy head branch name to clipboard