Fix a token rotation bug where a bot token is not refreshed if a user token does not exist

[seratch]

This pull request fixes a token rotation feature bug where a bot token cannot be refreshed if a user token does not exist in the same installation. This bug can affect all token-rotation enabled apps using the built-in InstallationStoreAuthorize. I overlooked this bug as I did tests only with installations that have both a bot and a user token 🤦

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

• slack_bolt.App and/or its core components
• slack_bolt.async_app.AsyncApp and/or its core components
• Adapters in slack_bolt.adapter
• Document pages under /docs
• Others

Requirements (place an x in each [ ])

Please read the Contributing guidelines and Code of Conduct before creating this issue or pull request. By submitting, you are agreeing to those rules.

• 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 ./scripts/install_all_and_run_tests.sh after making the changes.

[codecov]

Codecov Report

Merging #437 (4e053ff) into main (f70e1f4) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #437   +/-   ##
=======================================
  Coverage   91.34%   91.34%           
=======================================
  Files         167      167           
  Lines        5497     5497           
=======================================
  Hits         5021     5021           
  Misses        476      476           

Impacted Files	Coverage Δ
slack_bolt/authorization/async_authorize.py	79.16% <100.00%> (ø)
slack_bolt/authorization/authorize.py	78.99% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update f70e1f4...4e053ff. Read the comment docs.