Fix #1369 ImageBlock title parsing defaults to incorrect behaviour when given a string

[seratch]

Summary

This pull request resolves #1369

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 #1374 (c45f93d) into main (3afc31a) will increase coverage by 0.01%.
The diff coverage is 92.30%.

❗ Current head c45f93d differs from pull request most recent head a6df459. Consider uploading reports for the commit a6df459 to get more accurate results

@@            Coverage Diff             @@
##             main    #1374      +/-   ##
==========================================
+ Coverage   85.55%   85.57%   +0.01%     
==========================================
  Files         111      111              
  Lines       11570    11582      +12     
==========================================
+ Hits         9899     9911      +12     
  Misses       1671     1671              

Impacted Files	Coverage Δ
slack_sdk/models/blocks/blocks.py	96.60% <92.30%> (-0.24%)	⬇️

... and 1 file with indirect coverage changes

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[zimeg]

Looks great and handles errors so well! Tested this without a title too and found no problems! 🚀