text: Title: Fix next_cursor Flag in SlackResponse
  Body: In the SlackResponse, the presence of the next_cursor is determined by the `_next_cursor_is_present` method. 
  In most APIs, the next_cursor is returned as an empty string ("") as seen in `_next_cursor_is_present`. 
  However, in some APIs, the next_cursor is returned as null. To handle this situation, I would like to make modifications to the `_next_cursor_is_present` method.
  
  example) slackbot.responses.list
  ### Category (place an `x` in each of the `[ ]`)
  
  - [x] **slack_sdk.web.WebClient (sync/async)** (Web API client)
  - [ ] **slack_sdk.webhook.WebhookClient (sync/async)** (Incoming Webhook, response_url sender)
  - [ ] **slack_sdk.models** (UI component builders)
  - [ ] **slack_sdk.oauth** (OAuth Flow Utilities)
  - [ ] **slack_sdk.socket_mode** (Socket Mode client)
  - [ ] **slack_sdk.audit_logs** (Audit Logs API client)
  - [ ] **slack_sdk.scim** (SCIM API client)
  - [ ] **slack_sdk.rtm** (RTM client)
  - [ ] **slack_sdk.signature** (Request Signature Verifier)
  
  ### Requirements
  
  Please read the [Contributing guidelines](https://github.com/slackapi/python-slack-sdk/blob/main/.github/contributing.md) and [Code of Conduct](https://slackhq.github.io/code-of-conduct) before creating this issue or pull request. By submitting, you are agreeing to those rules.
  