improved doc: always return null and don't throw exception
improved doc: always return null and don't throw exception

graphql-java/graphql-java,blevandowski-lv/graphql-java,dminkovsky/graphql-java,blevandowski-lv/graphql-java,graphql-java/graphql-java,fderose/graphql-java,fderose/graphql-java,dminkovsky/graphql-java