fix start_cursor request type (issue:#498)

[Sho-ki]

Issue on #498

Description

This PR resolves a type mismatch issue encountered when using the start_cursor in database.query.
The current implementation leads to a conflict between string | undefined and string | null in various scenarios involving the cursor.

Changes Made

Updated the type for start_cursor in QueryParameters to accept both null and undefined.

Expected Behavior:

With this change, you will no longer face typing issues when assigning next_cursor from a query response to a variable.

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

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

    cursor = next_cursor  // Now this works without typing issues
    entries.push(...results)

    if (!has_more) {
      break
    }
  }

  return entries

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[corradopetrelli]

I also noticed this inconsistency. UP! 😄