fix GraphQLWrappingType

GraphQLNonNull wrapper can only be applies to nullable types

related: graphql#3597
see also: graphql#3617 (comment)

Author: yaacovCR

src/type/definition.ts

@@ -395,7 +395,7 @@ export class GraphQLNonNull<T extends GraphQLNullableType> {
 
 export type GraphQLWrappingType =
   | GraphQLList<GraphQLType>
-  | GraphQLNonNull<GraphQLType>;
+  | GraphQLNonNull<GraphQLNullableType>;
 
 export function isWrappingType(type: unknown): type is GraphQLWrappingType {
   return isListType(type) || isNonNullType(type);