ActionController::UrlGenerationError: No route matches {:action=>"raw", :controller=>"snippets/blobs", :path=>"new_snippet_file", :ref=...

There are reasonably frequent errors seen in Sentry that appear to be due to SnippetPresenter#raw_url building a raw URL for a PersonalSnippet with no ref.

https://sentry.gitlab.net/gitlab/gitlabcom/issues/2707621/?referrer=gitlab_plugin

ActionController::UrlGenerationError: No route matches {:action=>"raw", :controller=>"snippets/blobs", :path=>"new_snippet_file", :ref=>nil, :snippet_id=>#<PersonalSnippet id:2144157 $2144157>}, possible unmatched constraints: [:ref]
Did you mean?  snippet_blob_raw_path
               project_snippet_blob_raw_url
               project_snippet_blob_raw_path
               namespace_project_snippet_blob_raw_url
  lib/gitlab/graphql/generic_tracing.rb:40:in `with_labkit_tracing'
    return yield unless Labkit::Tracing.enabled?
  lib/gitlab/graphql/generic_tracing.rb:30:in `platform_trace'
    with_labkit_tracing(tags, &block)
  lib/gitlab/graphql/generic_tracing.rb:40:in `with_labkit_tracing'
    return yield unless Labkit::Tracing.enabled?
  lib/gitlab/graphql/generic_tracing.rb:30:in `platform_trace'
    with_labkit_tracing(tags, &block)
  app/graphql/gitlab_schema.rb:43:in `multiplex'
    super(queries, **kwargs)
...
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