connectionFromPromisedArray and PageInfo spec

[MarshHawk]

Hi all,

When connectionFromPromisedArray resolves from an empty array, startCursor and endCursor are null. However, the Connection spec specifies non-null:

          "name": "startCursor",
          "type": {
            "name": null,
            "kind": "NON_NULL",
            "ofType": {
              "name": "String",
              "kind": "SCALAR"
            }
          }
        },
        {
          "name": "endCursor",
          "type": {
            "name": null,
            "kind": "NON_NULL",
            "ofType": {
              "name": "String",
              "kind": "SCALAR"
            }
          }
        }

e.g.:

const conn = await connectionFromPromisedArray(
        Promise.resolve([]),
        {first: 2}
      );

which yields the following for conn:

{
  edges: [],
  pageInfo: {
    startCursor: null,
    endCursor: null,
    hasPreviousPage: false,
    hasNextPage: false
  }
}

[IvanGoncharov]

Closing as duplicate in favor of #243