fix: add articleId validation to avoid potential NPE in detail API

[Yummyyh]

This PR adds input validation for articleId in the detail API of ArticleRestController.

Changes:

• Added validation to ensure articleId is non-null and positive.
• Prevents potential NullPointerException when articleService returns null or invalid data.
• Improves API stability and aligns with common input validation best practices.

Effect:
This enhancement avoids unexpected runtime errors and makes the API more robust when receiving invalid client parameters.