Fix runtime error when aggregating results with error-containing left-value

[namenu]

The query below causes an infinite wait when executed.

query HangingQuery($id: ID!) {
  node(id: $id) {
    ... on Post {
      ... on Post {
        author {
          alwaysError
        }
      }
      author {
        name
      }
    }
  }
}

On the other hand, if you reverse the order of the field declarations, it will not.

query HangingQuery($id: ID!) {
  node(id: $id) {
    ... on Post {
      author {
        name
      }
      ... on Post {      # change the order
        author {
          alwaysError
        }
      }
    }
  }
}

It's worth noting that the fragment must be used to reproduce the error.

I guess in the process of doing merge-selected-values,
even though the left-value value is nil (which is resolved by alwaysError),
update to nil is called (which cause runtime error).

This PR fixes this issue.

[CLAassistant]

All committers have signed the CLA.

[hlship]

I'm sorry that I haven't kept track of this. I'd like to create a 1.2.2 release, but looks like your patch will need to be updated. If you can put that together, I'd be happy to merge.