Only add the original author if there isn't a linked GitLab account

CHANGELOG

lib/gitlab/github_import/base_formatter.rb

lib/gitlab/github_import/comment_formatter.rb

lib/gitlab/github_import/issue_formatter.rb

lib/gitlab/github_import/pull_request_formatter.rb

spec/lib/gitlab/github_import/comment_formatter_spec.rb

spec/lib/gitlab/github_import/issue_formatter_spec.rb

spec/lib/gitlab/github_import/pull_request_formatter_spec.rb