[Fusion] Fix null being returned for batch item

[tobias-tengler]

Given the following query

query {
  userBySlug(slug: "me") { # Subgraph 1
    feedbacks { # Subgraph 2
      edges {
        node {
          feedback {
            buyer {
              relativeUrl # Subgraph 1
              displayName # Subgraph 3
            }
          }
        }
      }
    }
  }
}

The Gateway will first go to Subgaph 1 to retrieve the User Entity, then it will go to Subgraph 2 to retrieve the list of Feedbacks, where each feedback is associated with another User.
For each User the Gateway should go to Subgraph 1 and Subgraph 3 in parallel to retrieve the necessary information.
However it creates a sequential query plan where it first does a batch call to Subgraph 1 and then with the results from Subgraph 1 a batch call to Subgraph 3. This is another issue though and I deemed it not worth fixing with Fusion v2 around the corner.

What this PR is addressing is the case if the batch call to Subgraph 1 returns null for some of the items being requested. This currently leads to an exception when building the variables necessary for the second batch call to Subgraph 3.
This exception will cause the requested fields from Subgraph 3 to be null for all User entities.

This PR makes it so that successful batch items are not affected by a missing one.
The effect of the fix can be seen in this diff: 7e240d8#diff-e3ebe93626ac479da605def8dda43ed76779f46f685c83342add3513c107ea03