Error when fetching past end of Launches

Cursor is defined as non-nullable:
```
type LaunchConnection { # add this below the Query type as an additional type.
cursor: String!
hasMore: Boolean!
launches: [Launch]!
}
```

But if no launches are retrieved (for example if you pass in a cursor for which `hasMore` was false), then the code will carefully return `null` for the cursor:

```
return {
launches,
cursor: launches.length ? launches[launches.length - 1].cursor : null,
```

Resulting in INTERNAL_SERVER_ERROR:

从 Query 类型下面的 LaunchConnection schema 和 issue 中所示的 resolver 返回对象开始。使用超出 launches 末尾的 cursor 重现一个请求，然后使 schema 和返回的 cursor 行为保持一致。完成的标准是该请求返回有效的 GraphQL 响应，而不是 INTERNAL_SERVER_ERROR。

由索引模型根据 Issue 内容生成。