fix(user): change slack_user_id to empty string before field change

[thenav56]

Addresses

• Null db constraint error

Changes

• Update null slack_user_id to empty string

This PR doesn't introduce any:

• temporary files, auto-generated files or secret keys
• n+1 queries
• flake8 issues
• print
• typos
• unwanted comments

This PR contains valid:

• tests
• permission checks (tests here too)
• translations

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.08%. Comparing base (3c92762) to head (22a70cc).
⚠️ Report is 2 commits behind head on develop.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #189   +/-   ##
========================================
  Coverage    87.08%   87.08%           
========================================
  Files          204      204           
  Lines        11807    11807           
  Branches      1004     1004           
========================================
  Hits         10282    10282           
  Misses        1141     1141           
  Partials       384      384           

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