Reuse default generated snippet name

What does this MR do?

If the user created a snippet without a file name, we will assign a name for the blob file like snippetfile1.txt. If we edit the snippet and remove the file name again, we will build the new name snippetfile2.txt, and so on.

Instead, we can reuse the existing one and avoid generating new names because it can be confusing.

Closes #211931 (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