When a new note is created for a merge request with no associated
metrics, Sidekiq will error out with `NoMethodError: undefined method
'first_comment_at' for nil:NilClass`.

This commit ensures a merge request metrics exists before attempting to
update it. There is still an outstanding question of why no such metrics
were created, but that can happen for older merge requests.

Closes https://gitlab.com/gitlab-org/gitlab/issues/197464