Add validations

[yamcodes]

Clear and concise description of the problem

Add validations like checking if a slug is not empty before creating an article.

Suggested solution

Validate using TypeBox.

Alternative

Not to do it.

Additional context

• Check if it's part of the RealWorld expectations: https://realworld-docs.netlify.app/specifications/backend/endpoints/#favorite-article
• Check if it's a part of the official RealWorld express app

Validations

• Read the Contributing Guide.
• Read the README.md.
• Check that there isn't already an issue that requests the same feature.

[yamcodes]

Fixed in #174