Import/export: ArgumentError: comparison of String with 0 failed

https://sentry.gitlab.net/gitlab/gitlabcom/issues/739851/

ArgumentError: comparison of String with 0 failed
  gitlab/import_export/project_tree_restorer.rb:132:in `>'
    level = @project.group.visibility_level if @project.group && level > @project.group.visibility_level
  gitlab/import_export/project_tree_restorer.rb:132:in `visibility_level'
    level = @project.group.visibility_level if @project.group && level > @project.group.visibility_level
  gitlab/import_export/project_tree_restorer.rb:110:in `project_params'
    attrs = json_params.merge(override_params).merge(visibility_level)
  gitlab/import_export/project_tree_restorer.rb:102:in `block in restore_project'
    @project.update(project_params)
  gitlab/timeless.rb:12:in `timeless'
    block.call
...
(77 additional frame(s) were not displayed)

ArgumentError: comparison of String with 0 failed