GraphQL mesh supergraph bug

Getting started

npm install
npm start

Reproduce the bug

1. Open the browser and go to http://localhost:4000/graphql
2. Run the following query:

   query {
       products(first: 1) {
           id
           name
           discounts {
               id
           }
           categories {
               id
               discounts {
                   id
               }
           }
       }
   }

3. You will receive the following error:

   {
       "data": {
           "products": [
           null
           ]
       },
       "errors": [
           {
               "message": "Cannot return null for non-nullable field Category.discounts.",
               "path": [
                   "products",
                   0,
                   "categories",
                   0,
                   "discounts"
               ]
           }
       ]
   }

4. Interestingly, the following query works:

       query {
           products(first: 1) {
               id
               name
               discounts {
                   id
               }
               categories {
               id
                   test: discounts {
                       id
                   }
               }
           }
       }