[13.x] Mark generation type field nullable in processColumns @return shape

[mosabbirrakib]

The processColumns shape declares generation: array{type: string, ...}, but three of the four concrete implementations have a default => null arm in their type match:

• MySqlProcessor line 61: default => null
• PostgresProcessor line 98: default => null
• SQLiteProcessor line 46: default => null

So generation.type is genuinely nullable. Tightening the parent shape to type: string|null to match.