Add presence validation to content and title in snippet rest endpoints

What does this MR do?

This MR adds presence validation to the fields content and title in the snippet create and update endpoints. At the moment, that validation is handled by the validations in the snippet model, but soon we will remove it and we want to still prevent the creation or update of snippets when those fields are empty.

Refs #218224 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content