Remove snippet content presence validation

What does this MR do?

This MR removes the content presence validation from snippets. Since now we can push content from git, we can push empty files to the repository. Therefore, it doesn't make sense to have those different limitations depending on where the info is coming from.

We're still leaving the requirements in the API in order to avoid creating empty snippets in an automated way.

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

Closes #209748 (closed)