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:

Start with the LaunchConnection schema below the Query type and the resolver return object shown in the issue. Reproduce a request using a cursor past the end of the launches, then make the schema and returned cursor behavior consistent. Done means the request returns a valid GraphQL response instead of INTERNAL_SERVER_ERROR.

Written by the indexing model from the issue text.