Make next_cursor extraction logic even more robust (ref #1407)

[seratch]

Summary

This pull request improves the internal logic to extract next_cursor from Slack Web API response. As of today, all the public APIs return empty string (meaning "") when the API does not have its next page. With that being said, the platform may add some inconsistent behavior in the future. Thus, just in case, we want to make this SDK code even more robust for such scenario. Refer to #1407 for its context.

Category (place an x in each of the [ ])

• slack_sdk.web.WebClient (sync/async) (Web API client)
• slack_sdk.webhook.WebhookClient (sync/async) (Incoming Webhook, response_url sender)
• slack_sdk.socket_mode (Socket Mode client)
• slack_sdk.signature (Request Signature Verifier)
• slack_sdk.oauth (OAuth Flow Utilities)
• slack_sdk.models (UI component builders)
• slack_sdk.scim (SCIM API client)
• slack_sdk.audit_logs (Audit Logs API client)
• slack_sdk.rtm_v2 (RTM client)
• /docs-src (Documents, have you run ./scripts/docs.sh?)
• /docs-src-v2 (Documents, have you run ./scripts/docs-v2.sh?)
• /tutorial (PythOnBoardingBot tutorial)
• tests/integration_tests (Automated tests for this library)

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.
• I've run python3 -m venv .venv && source .venv/bin/activate && ./scripts/run_validation.sh after making the changes.

[codecov]

Codecov Report

Merging #1409 (4d58b5b) into main (2caacbb) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1409      +/-   ##
==========================================
- Coverage   85.47%   85.46%   -0.01%     
==========================================
  Files         111      111              
  Lines       11857    11857              
==========================================
- Hits        10135    10134       -1     
- Misses       1722     1723       +1     

Files	Coverage Δ
slack_sdk/web/internal_utils.py	82.72% <100.00%> (ø)

... and 1 file with indirect coverage changes

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