fix(creator): guard empty slug params consistently

[shoaib050326]

Summary

• normalize empty and whitespace-only creator slug params before deeper schema handling
• return the same Creator ID is required validation error for empty-like creator slug input
• add a focused schema regression test for empty, whitespace-only, and valid creator IDs

Root Cause

The creator slug preprocessing only treated the exact empty string as missing input. Whitespace-only route params were left to deeper validation, and exact empty strings surfaced a generic Zod Required error instead of the creator-specific validation message.

Testing

• pnpm exec ts-node src/modules/creator/creator-profile.schemas.test.ts
• pnpm exec ts-node src/modules/creator/creator-list-page.guard.test.ts

Closes #80

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