feat(creator): normalize empty slug params

[kryputh]

Summary

• add a narrow helper that normalizes empty-string creator slug input before route validation
• reuse that helper directly in the creator profile params schema
• keep behavior predictable by only converting exact empty-string values to undefined

Verification

• pnpm build
• pnpm exec eslint src/modules/creator/creator-slug-input.utils.ts src/modules/creator/creator-profile.schemas.ts
• pnpm exec ts-node --transpile-only -e "const { CreatorProfileParamsSchema } = require('./src/modules/creator/creator-profile.schemas'); const empty = CreatorProfileParamsSchema.safeParse({ creatorId: '' }); const valid = CreatorProfileParamsSchema.safeParse({ creatorId: 'creator-slug' }); console.log(JSON.stringify({ emptySuccess: empty.success, emptyIssues: empty.success ? [] : empty.error.issues.map((issue) => ({ path: issue.path.join('.'), message: issue.message })), validSuccess: valid.success, validValue: valid.success ? valid.data.creatorId : null }, null, 2));"

Closes #84

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