Inconsistent type: Cursor for parameters is string|undefined but for responses is string|null

[jcbages]

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Describe the bug
When calling something like database.query that receives a cursor as a parameter, its type is string | undefined as it's start_cursor:?string. However, the returned cursor from the response is of type string | null. This causes issues when looping to obtain multiple pages while reassigning the cursor variable as the initial typing of the cursor variable will conflict with one of these.

I checked the type definitions and this is the case for all calls that receive and return a cursor. This is easy to workaround but it degrades the typing experience a little.

To Reproduce
Node version:
Notion JS library version:
2.2.14

Steps to reproduce the behavior:

let databaseId = '<some_id>';
let entries = [];
let cursor: string | null = null;
let entries = [];

while (true) {
    const { results, has_more, next_cursor } = await this.notion.databases.query({
        database_id: databaseId,
        start_cursor: cursor
    });

    cursor = next_cursor;
    entries.push(...results);

    if (!has_more) {
        break;
    }
}

return entries;

Expected behavior
Expected: No typing issues
Got: Type 'string | null' is not assignable to type 'string | undefined'.

If we change the type of the cursor to be string | undefined then the error happens at cursor = next_cursor saying: Type 'string | undefined' is not assignable to type 'string | null'.

Additional context
Happy to make a PR for this :)

[Sho-ki]

I made a PR for this issue. #510