Update validator to allow 0 board name

[theoholl]

• Resolves: can't create Board with title 0 #7370
• Target version: main

Summary

• Extend BaseValidator class to allow the string value "0" as the board title.
• Add unit test for the BoardService.
• Update test function in BoardMapperTest to be more robust against database changes introduced from outside the test.

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Documentation (manuals or wiki) has been updated or is not required

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