Fix snippet url helper when repository is not present

What does this MR do?

In the route helper gitlab_raw_snippet_blob_url, when the ref argument is not passed, we use the snippet.repository.root_ref but, what happens if the snippet does not have a repository? That command will return nil and the helper will fail.

In this MR we're fixing that by calling snippet.default_branch instead of snippet.repository.root_ref. In this method, we call snippet.repository.root_ref and, when the repository does not exist, we return the instance default branch.

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Related to #335636 (closed)