Handle Snippet file name errors in backfill

What does this MR do?

We need to modify the snippet backfill migration to handle snippets with file names that cause an error in Gitaly due to being unable to be parsed into valid git paths.

To do that, we are going to parametize the file name if it raises an error, or use the default (e.g. snippetfile1.txt) if that parameterised value is blank.

References #216273 (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

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.