fix: update remaining GraphQLNonNull<GraphQLType> codepoints to

`GraphQLNonNull<GraphQLNullableType>`

This is related to graphql#3597, in the sense that graphql#3597 made the
`GraphQLNonNull<GraphQL*Type>` => `GraphQLNonNull<GraphQLNullable*Type>`
change in other code points related to the `isNonNullType` function, but
not within `GraphQLWrappingType` or assertNonNullType.

This PR was prompted by the uncovering of a bug by a different PR, see:
graphql#3617 (comment)

Author: yaacovCR

src/type/definition.ts

@@ -183,7 +183,9 @@ export function isNonNullType(
   return instanceOf(type, GraphQLNonNull);
 }
 
-export function assertNonNullType(type: unknown): GraphQLNonNull<GraphQLType> {
+export function assertNonNullType(
+  type: unknown,
+): GraphQLNonNull<GraphQLNullableType> {
   if (!isNonNullType(type)) {
     throw new Error(`Expected ${inspect(type)} to be a GraphQL Non-Null type.`);
   }
@@ -395,7 +397,7 @@ 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);