Kotlinify codebase

[azrael8576]

• Remove unnecessary nullable types
• Replace no-op method bodies with Unit
• Convert to expression body
• Replace if with when
• Remove braces from 'when' entries
• Remove braces from if statement
• Convert to single line lambda
• oneline if/returns
• Replace 'contains' call with 'in' operator

Following this refactor, it could be great to envision a more "strict" code formatter like ktlint 1.0.1 (we are currently stuck at 0.48.1)

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Fixes #89 🦕