Fixes a runtime error that occurs when deep-merging fragments

[namenu]

Fixes a bug where inconsistent results occur based on the fragment's declared order.

I fixed a similar issue in #_453, but in that case the fragment was nested, whereas this case is not.

This fails:

query MyQuery {
  node(id: "1000") {
    ... on Post {
      id
      ...PostFragment
    }
  }
}

fragment PostFragment on Post {
  author {
    alwaysFail
  }
  ...PostFragment2   # <--
}

fragment PostFragment2 on Post {
  author {
    name
  }
}

While this is not:

query MyQuery {
  node(id: "1000") {
    ... on Post {
      id
      ...PostFragment
    }
  }
}

fragment PostFragment on Post {
  ...PostFragment2   # <--
  author {
    alwaysFail
  }
}

fragment PostFragment2 on Post {
  author {
    name
  }
}

My guess is that deep-merge doesn't handle the situation where nil comes in instead of map.
It can be ::null, or it can actually be nil.
It depends on whether the object that catches bubbled-up error is non-null or not.

[devgrapher]

그냥 요러케 고쳐보면 어떨까싶은 느낌이 있습니다.. nil또는 null을 반환하게..

(cond
    (null? left)
    left

    (null? right)
    right
...

null이 반환되는 경우에는 애초에 null이 들어왔다는건데, 이건 보통 필드리졸버가 null을 임시로 넣어두고 나중에 null-collapser로 nil처리가 될거같고요,
nil이 애초에 들어왔다면 이미 null-collapser를 통과한 값이거나 unauthorized 상황인거라 그냥 nil 반환해주면되는거 아닐까 싶은 생각인데.. 테스트를 좀해봐야. ㅎㅎ

[namenu]

#12